Mustard algae: the yellow dust that always comes back
- A yellow-beige deposit like sand or pollen, always resettling in the same shady spots: mustard algae.
- It resists everyday chlorine levels — a standard shock isn't always enough.
- The treatment combines brushing, heavy chlorination and disinfecting EVERYTHING that touches the water (net, brush, swimwear).
- Skip the "equipment" step and recontamination is all but guaranteed.
A yellow-beige deposit, fine like dust or pollen, that billows up in a cloud when you brush it and always returns to the same spots (shady areas, steps, the floor): that’s mustard algae. What makes it special — and so maddening — is that it tolerates everyday chlorine levels. Normal maintenance doesn’t kill it, and a standard shock sometimes only pushes it back. The treatment that works combines brushing, chlorination well beyond a classic shock, and — the step everyone forgets — disinfecting all your equipment. The doses depend on your pool: the CliniPool app calculates them.
Why it always comes back
Mustard algae has two superpowers. First, chlorine resistance far above that of green algae: at everyday maintenance levels, it survives quite happily. Second, an ability to cling to everything: walls, brush, net, robot cleaner, ladder, swimwear. You treat the pool, it comes back via the net. That’s why this treatment is as much about method as it is about product.
The full treatment
1. Brush everything, methodically. Walls, floor, steps, corners, waterline. Dislodged, suspended algae are far more vulnerable to chlorine than algae stuck to a wall.
2. Test the water and correct the pH. As with any treatment, a high pH disarms the chlorine before it gets to work. Check the stabiliser too: if it’s too high, it often explains why previous shocks failed.
3. Heavy chlorination. This is where mustard algae stands apart: it demands a concentration well above a classic shock, held long enough. The exact maths — based on your volume, your product and your current level — is the app’s job, safeguards included.
4. Disinfect EVERYTHING that touches the water. Brush, net, vacuum hose, robot cleaner (and its filter), ladder, toys — and machine-wash the swimwear. Drop the small equipment into the pool during the heavy chlorination: it gets treated along with the water. Skip this step and you’ll reseed your pool by tomorrow.
5. Continuous filtration + a second brushing one to two days later: the treated areas release the last spores, and the still-elevated chlorine finishes them off.
After the treatment
Keep an eye on the usual spots for two weeks: a quick reappearance points either to a forgotten piece of equipment (the robot cleaner is the classic suspect) or to a high stabiliser that weakened the chlorination. The app keeps your test history and warns you if the conditions for a comeback are settling back in.
Frequently asked questions
How do I tell mustard algae from pollen or sand?
Pollen tends to float on the surface and arrives in episodes (season). Sand doesn't dissolve when you rub it. Mustard algae billows up in a yellow cloud when you brush it, then resettles in the same spot within hours.
Why does it come back after treatment?
Two reasons: an underdosed treatment (it tolerates standard chlorine levels) or recontamination from undisinfected equipment — brush, net, robot cleaner, even swimwear.
Is mustard algae dangerous?
It isn't toxic in itself, but its presence signals inadequate sanitisation, and surfaces become slippery.