Mustard Algae That Comes Back (When Levels Are Good)
You're seeing: Yellow/mustard (or green-yellow) dust on shaded walls and floor that brushes away and returns.
Do a normal SLAM to completion, then raise free chlorine to the extra-high mustard level for your CYA (~1.5× SLAM) for 24 hours — and while you're there, disinfect everything that touched the water: brushes, poles, toys, floats, ladders, light niches, and swimsuits (run them through the wash). Miss one item and it re-seeds the pool.
Mustard (yellow) algae is the frustrating one: you SLAM the pool, it goes away, and days later a yellow-ish dust is back on the shaded walls. That’s because it tolerates normal chlorine and hides on surfaces — and on everything that touches the water.
First: is it actually mustard algae?
Most of the time it’s pollen, not algae. Mustard algae grows on walls and floor, especially the shaded side, and once you brush it at shock level it stops coming back while you hold that level. Pollen settles only on the bottom, shows up everywhere, and keeps accumulating even at shock level. If brushing at maintained high chlorine makes it stop returning, it was algae.
Do this (the treat-everything protocol)
- Run a normal SLAM to completion first — all three exit criteria.
- Then raise FC to the mustard level for your CYA — about 1.5× your SLAM level (roughly CYA 40 → ~24, 50 → ~29, 60 → ~35) — and hold it for about 24 hours. The calculator gives the dose.
- While you’re at that level, disinfect everything that touched the water: brush the entire pool again; wipe down or soak (an hour in the pool) all poles, brushes, nets, vacuums, toys, and floats; pull and brush underwater light niches; run any fountain/waterfall for an hour; and launder every swimsuit that’s been in.
- Be thorough. One untreated float or brush re-seeds the whole pool and you start over.
- After 24 hours, let chlorine drift back to the normal target for your CYA — and keep it there so it can’t re-establish.
Enter your CYA to get the exact number to hold:
Your mustard-algae SLAM level
Enter your CYA to get the SLAM level to hold while you clear mustard algae (then keep FC a hair above target afterward).
Straight off Trouble Free Pool's FC/CYA chart (minimum ≈ 7.5% of CYA, SLAM ≈ 40%), snapped to the nearest published row — the chart table stays the source of truth. Exact doses for your gallons: the chemical calculator. CYA way over 100? More chlorine isn't the fix — lowering CYA is.
What NOT to do
Don’t reach for “mustard algaecide” as the primary fix — it’s the treat-everything thoroughness and the elevated chlorine that kill it, not a specialty bottle. Don’t stop at a normal SLAM (it always seems to work, then it’s back). And don’t skip the tools/toys/swimsuits step — that’s the whole reason it recurs.
If your algae is plain green rather than yellow, start with My Pool Is Green. If it “looks fine” chemically but keeps growing, it’s usually high CYA. If instead it’s dark spots that stay put and regrow after brushing, that’s a different beast — black algae.
Frequently asked
Why does my mustard algae keep coming back?
Mustard (yellow) algae tolerates normal chlorine levels and hides on shaded surfaces, so it clears during a SLAM and returns as soon as chlorine drops back to normal. It also survives on anything that contacted the water — brushes, poles, toys, floats, ladders, underwater light niches, swimsuits. If you don't disinfect all of those at the same time you raise chlorine, the survivors re-seed the pool.
What chlorine level kills mustard algae?
Higher than a normal SLAM — roughly 1.5× the SLAM level for your CYA (about 57–60% of CYA). For example around CYA 40 that's ~24 ppm, and around CYA 60 it's ~35 ppm, held for about 24 hours after a completed normal SLAM. Use liquid chlorine and a FAS-DPD test.
Is it mustard algae or just pollen?
Most 'mustard algae' is actually pollen. Tells: mustard algae grows on walls and floor (especially the shaded side) and, once brushed at shock level, won't return while you hold that level; pollen settles only on the bottom, appears everywhere, and keeps accumulating even at shock level. If brushing at a maintained high FC makes it stop returning, it was algae.