I drained the pool and it appears that the stains are not really stains. I think it is yellow algae that is growing in the crevices and round pocket holes you can't reach. Even so, I tried applying ascorbic acid and then scrubbing it to no avail. If anything, it bleached the area some from the natural white wall color (looks a little greyer to me).

I would then assume that once I get the pool full again with the correct ph, TA, CH, and CYA, I can keep the chlorine levels up to get rid of the algae.

Greg