I had to drain my pool down yesterday to lower my CYA. While the water was down I took some pictures of the side wall. When the water was full, I thought it looked like a mustard algae or something similar. Now that I see it with no water, I'm not sure. My pool is 28 years old and yes, it's the original plaster. We plan on re-plastering or pebble tech within the next two years, but it's not in the budget right now. Are these blotches a stain of some sort, or is it just the plaster wearing thin? Yesterday prior to the drain, my numbers are listed below.
Water temp 68 deg