I bought the house 3 months ago, so I dont have a full picture. I used the trichlor tabs that the previous owner used before. Do not believe that there is any copper in it. Have not used any algaecide.Honestly looks like a metal stain. Copper would be one. Any use of copper based algaecides or trichlor pucks with copper in them?
Have stopped since and did a simultenous water exchange of 20k gallons to try to get the CYA down...and have not used anything in the pool except liquid chlorine. Only used nylon and wire brushes.
Pool had no staining before my pump broke and the algae took over.