Chlorine shock and a stiff brush are the only way.

Yes. Mustard algae is a pain because it’s chlorine-resistant and loves to hide in low-circulation spots. You can’t scrub it off with a magic potion—you need to blast it with high chlorine and physically break it loose.

Raise your chlorine to shock level (3–5x normal for that algae type), brush every inch of the liner—corners, behind ladders, under the skimmer—and run the filter continuously. Backwash or clean the filter when pressure rises. Some algaecides labeled for mustard algae help, but chlorine does the heavy lifting.

You might need to repeat the shock-and-brush cycle a few times. The liner is delicate, so no wire brushes or abrasive pads—use a nylon pool brush.

Don’t skip the brushing. That’s where most people fail.