A ten-second brush test tells the three pool algae apart, and each one needs a different treatment. Green algae clouds the water, mustard algae lifts off a shaded wall in a cloud, and black algae stays put as raised spots in the plaster.
The type matters because the wrong treatment is expensive. Treating black algae like green wastes a month and a lot of chlorine, and treating mustard like green clears it for four days before it returns to the same shaded wall.
The treatments differ, but the cause doesn’t. All three grow when free chlorine runs too low for too long, and the most common reason we see is cyanuric acid (CYA), sold as stabilizer, drifting high. The CDC’s home pool guidance warns that “disinfection times are longer in the presence of cyanuric acid.” So a pool can show a normal chlorine reading while that chlorine works too slowly to keep algae out.
Identify it in ten seconds
| Green | Mustard | Black | |
|---|---|---|---|
| Where it is | Everywhere, suspended in the water | Shaded walls, steps, behind ladders | Plaster, grout lines, corners, rough spots |
| Color | Green tint through the whole pool | Yellow, tan, or brownish | Blue-black or dark green spots |
| Looks like | Cloudy or opaque green water | Dirt or sand that won’t vacuum away | Discrete spots, pencil eraser sized or smaller |
| Brush test | Water is already green, nothing to brush | Lifts easily in a cloud, then resettles | Does not lift. You can feel it raised |
| Water clarity | Poor to none | Usually clear | Usually clear |
| How fast it spreads | Days | Days on the same surfaces | Weeks, slowly, but it deepens |
| Chlorine resistance | Low | Moderate | High, it has a protective outer layer |
The brush test is the whole diagnosis. Take a pole brush, work a small area hard, and watch. Material that lifts into a cloud and then settles back over the next hour is mustard. A spot that stays exactly where it was, with a slightly raised, crusty feel, is black. Green water answers the question on its own.
Two things that aren’t algae
Pollen. In March and April, a yellow film on the surface and along the waterline is usually oak or elm pollen rather than mustard algae. It floats rather than clinging, it arrives all at once, and it skims off.
Metal staining. Brown, gray, or greenish staining that is smooth, flat, and under the surface rather than on it, and that doesn’t brush at all, is usually iron or copper rather than algae. If a vitamin C tablet held against it lightens the spot, it’s a metal stain and chlorine will make it worse rather than better.
Green algae
Green algae is the common one, the fast one, and the one with the simplest fix. Free chlorine reached zero, and in a North Texas summer the pool followed within two to three days. Our green pool guide covers the recovery sequence in full, and what green pool recovery costs in DFW covers the pricing if you hand it over.
Recovery runs in four steps. Balance pH first so chlorine can work, then shock to a level set against your cyanuric acid rather than a fixed number. Brush everything, and run the filter continuously. Expect the water to go cloudy gray before it goes clear, which is progress rather than a setback.
Mustard algae, which is our local recurrer
Mustard algae, often a form of green algae growing in a different habit, fools people reliably in North Texas. It tolerates chlorine levels that would kill green algae. It settles on the shaded side of the pool, typically the north or east wall, behind ladders, under steps, and in the dead spot the return jets don’t reach. It looks exactly like a patch of dirt or fine sand, which is why owners vacuum it, watch it disappear, and find it back in the same place two days later.
Why it recurs
- Mustard algae survives on surfaces outside the pool: on a pole brush, a leaf rake, a vacuum head, a float, or a swimsuit. Treating the water without treating the equipment reseeds the pool.
- It hides in the filter, particularly in a cartridge that hasn’t been cleaned during treatment.
- It lives where circulation is poor, so it survives the same chlorine level that clears the rest of the pool.
Treating it so it stays gone
- Balance pH to the lower end of range so chlorine is at its most effective.
- Brush the affected surfaces hard, every day, including behind and underneath everything.
- Shock to a level appropriate to your cyanuric acid and hold it for several days rather than dosing once. Holding it is the part people skip.
- Clean the filter during treatment, not after. A cartridge that is holding mustard is a reservoir.
- Put every piece of pool equipment in the shallow end during the shock: brushes, poles, vacuum heads, floats, toys. Anything porous that can’t go in should be replaced.
- Improve circulation where it settled. Aiming a return jet at the dead corner does more long-term good than another dose of anything.
Mustard algae products work, and better alongside those six steps than instead of them.
Black algae
Black algae, strictly a cyanobacterium, is the only one on this page that takes weeks to clear rather than days. It grows roots into plaster and gunite and forms a waxy outer layer that chlorine struggles to penetrate. That combination is why it survives a shock that clears everything else, and why it comes back from what looks like a cleared spot: the visible part was killed and the roots weren’t.
It favors rough spots, grout lines, corners and cracks, so it is more common on older, worn plaster than on a recently resurfaced pool. An aging pool with black algae has two problems, and one of them is the surface.
What works on black algae
- Mechanical destruction of the protective layer. A stainless steel brush on plaster or gunite only, never on vinyl or fiberglass, worked hard over every spot. Everything else depends on this step, and no chemical substitutes for it.
- Direct application to the spots. A trichlor tablet rubbed on individual spots after brushing puts concentrated chlorine exactly where the roots are.
- Sustained elevated chlorine across the whole pool for days, not a single dose.
- Repetition. Brush and treat every few days for weeks. Each cycle takes out another layer.
- Patience about the marks. Dead black algae frequently leaves a stain behind in the plaster. A gray or brown mark that no longer feels raised and no longer spreads is dead, and it may not fade until the next resurface.
If black algae keeps returning after proper treatment
The surface is usually the reason. Plaster runs 7 to 12 years, and worn, porous, or crazed plaster gives roots somewhere to hide that no brush can reach. At that point the useful conversation is about resurfacing, not another chemical. DFW replastering commonly runs $6,000 to $15,000, and equipment lifespan in Texas heat covers where a surface sits in the wider replacement picture.
The shared cause, and how to prevent it
All three trace to free chlorine that was too low for too long. The CDC puts the floor at 1 ppm of free chlorine, and at least 2 ppm in a pool using cyanuric acid, which covers most pools here. In this market, chlorine runs low for four usual reasons.
- Cyanuric acid drifted high and the chlorine target wasn’t raised to match. The most common cause we see, and invisible on the chlorine test. The CYA guide explains the ratio, and liquid versus tabs covers the product choice that causes it.
- Circulation failed. A pump that lost prime, a clogged basket, or a loaded filter. Chemistry can’t fix a pool that isn’t moving.
- A gap in attention during summer. A vacation, or an interval that was fine in April and not in August. Weekly service in Texas covers why the interval matters more here.
- A storm. Heavy rain dilutes chlorine and stabilizer, drops pH, and washes organics in, all at once.
Fix the cause or you’ll be reading this page again next season. Prevention is cheaper than any treatment above:
- Hold free chlorine against your cyanuric acid level rather than a fixed number: at least 7.5 percent of it on a chlorine pool and 5 percent on salt.
- Brush at every visit, including the shaded wall and behind the ladder, because brushing is what stops algae establishing rather than what removes it once it has.
- Keep the filter clean and the pump schedule long enough for a full daily turnover.
- Test twice a week from May through September rather than once.
- Deal with storms the same evening, not the following weekend.
What we would take on
Green and mustard are both routine work. A green pool recovery handles the first, and mustard usually clears within weekly pool service, because its treatment is as much circulation, brushing and equipment sanitation as chemicals.
Black algae on an aging surface is the one where we would rather set expectations before starting: weeks of repeated work, a likely permanent stain, and a real possibility that the surface is the problem. That isn’t a pleasant quote to give, and it’s better than selling four rounds of a product that couldn’t reach the roots.
The brush test decides which of those conversations you’re having, so do it before you buy anything.
