A(nother) friend's pool plaster is pitted and looks stained - when they asked me to come over, the pH was very low. Tested everything else with the TFTK and all but pH was in range.
We had a fire last year and a lot of pools got ash, and he said he tried to clear up the algae with Cl, but wound up using an algacide.
My initial impression was that the pH was too low, which could affect the plaster, but after looking closer, there's definate color in these stains:
Haven't tried anything yet...