Plaster pools are commonly acid washed to address issues like yours. Acid washing works by dissolving the top layer of plaster, and exposing a 'new' top layer underneath.

BUT . . . acid washing does not evenly dissolve that top layer, and the resulting plaster is rougher and more susceptible to stains, than the original surface. Careful, skilled application can reduce the problem, but many contractors either aren't skilled, or can't be bothered since their customers don't know the difference.

AND . . . the effects of acid-washing a pool with COLORED plaster can be unpredictable. Personally, I don't have enough experience with colored plaster to know how to tell whether a colored plaster pool can be acid washed or not.