Hi I have lurking this forum for a few years, lots of good advice here. Heres my problem. I have had my in ground salt water, plaster pool for about 9 years now, very few problems. This winter I let it get away from me. My salt cell went bad, and before I knew it I had mustard algae. I cant seem to get rid of it. What really bothers me, is that is seems like its rooting into my plaster walls. I scrape it hard with an algae brush, but I still see big dull yellow stains in my carribean blue plaster. I have tried raising chlorine levels and I also use Orenda a phophate remover . It keeps coming back. Mabee I am not raising clorine levels enough. Please help with some advice, and a way to unstain my pool walls. I have a big 25Lb bucket of tricolor 99% powder recently bought to uses for shocking occasionally. Heres the specs on my pool.

21,000 gal inground gunite

PH 7.6
Free Cl 3-5
Alk 150
CY 100
45 PPM Borax