Bought this house two years ago. Re-plastered pool at that time. Converted over to using liquid bleach sanitizing program last summer thanks to wonderful folks here. In Houston and pool is open all year. Pool has never been used since re-plaster (I know, I know - long story). Have been noticing what "looks" like algae where rounded half-circle steps meet pool wall and on plaster where upper tile meets plaster, and a few other areas on sidewalls. None of the floor of pool has these discolorations.
Yellowish-greenish-brownish stains that are similar in appearance to mustard algae (kind of). Stains do not brush off whatsoever. Have done two overnight chlorine tests with zero chlorine loss. Ph, FC, TC, CH, etc have been kept at near perfect levels. CYA is a bit high at 70. Did a 60% drain last year to get CYA down out of the ozone (100+ reading). Keep inline chlorinator with pucks on a 1 out of 5 setting (very lowest setting) to supplement bleach just a smidge.
I don't think these stains are organic because of their locations. I do not have well water. I use the Taylor-2006 test kit. Have not tested water for minerals. I have attached a couple of photos and have circled the areas where the majority of these stains are. I don't intend for you to be able to see the stains in these pics per se, just the geographical locations in the pool. Although the arrow does point to one stain that might be visible to you. Stains have been there since last September but then were barely visible to me. They have become increasingly obvious over the winter and are now hugely noticeable. Same areas of the pool all this time. No new or additional stains since these specific ones started last year.