We just had a pool build and plaster (Sunstone pebble) was done last week. it was acid washed Wednesday and they started filling up same day. Pool was filled Thursday. Saturday we get the maintenance kit and start brushing and I notice an area by the stairs that has brown stuff that I cannot brush away. Today, a week after pool started filling, I still cannot

brush it away. I of course let the builder know and they will come look at it. But can anyone give any insight on this and what should be done?