I'm new pool owner since may of this year.
When we bought the house, the pool was already running and after reading on this site and some other literature I had no problem keeping it running for the whole summer (never had any algae problems btw).
Now the problem: I have attached three photos where you can see brown color thing that looks like washed out sediment or something on the bottom of the pool (white plaster). I spotted it a couple of days ago but was busy to address it immediately. So today I got very surprised when I could't remove it with 8" aluminum algae brush.
Please help me to identify what it is and how to get rid of it .
Helpful info: 1. The pool is surrounded by trees, shrubs and all kinds of flora so there is always leaves and tree debris getting into the pool.
2. Test results with Taylor FAS-DPD : FC: 4.5 ppm, CC: 0 ppm, pH: 7.6, TA: 90 ppm, CH: 200 ppm, CYA: 50 ppm.
Thank you very much.