In the last month or so, I am noticing these stains in my gunite/concrete pool. They are more prominent in the corners (the angled area around the pool floor - where the wall meets the floor. The stains look brownish and dirt like (almost like dirt particles stuck on the concrete). I tried scrubbing them with a brush but they don't come off. Water test does not show any metal presence. Chemical balance looks good (except chlorine is a little high).
I have attached pics below.
What causes these stains and what can I try to get rid of them?