Mustard algae needs more than chlorine.

If you’ve been shocking and it’s back in a day or two, the problem is likely that your chlorine level wasn’t high enough for long enough. Mustard algae is a stubborn strain that laughs at routine shocking.

The fix: brush every surface (including walls, steps, and under the ladder) to break the algae’s film, then hit the pool with a heavy dose of chlorine—like, double the normal shock level—and keep it there for 24 hours. A good algaecide specific to mustard algae helps too. Also check your filter; if it’s running dirty, you’re just recirculating the problem.

One more thing: phosphates in the water feed algae. If you keep getting it back, test for phosphates and treat accordingly. Otherwise you’re fighting a losing battle.

Don’t half-ass the brushing. That’s where most people lose.