Mustard algae
Also called: yellow algae.
Mustard algae is a yellow, dusty algae that grows on walls and floor, especially the shaded side, tolerates normal chlorine levels, and clears during a SLAM only to return when chlorine drops back to normal. It is resistant to chlorine, not immune.
Why it matters
Most "mustard algae" is actually pollen: pollen settles only on the bottom and keeps accumulating even at shock level. For the real thing, run a normal SLAM to completion, then raise free chlorine to about 1.5× your SLAM level and hold it for about 24 hours. While you're there, disinfect everything that touched the water, including brushes, poles, toys, floats and swimsuits, because one missed item re-seeds the pool. It lives in dead spots where circulation never reaches, so aim your returns and brush those corners hardest, and vacuum the dead algae to waste. Skip sodium bromide and copper.
In numbers
Around CYA 40 the mustard level is ~24 ppm, and around CYA 60 it's ~35 ppm, held for about 24 hours after a completed normal SLAM.