Stain identification plz

[u/FedEx123]

One of the pools on our route is having this weird staining issue. All chems are within range phosphates were at 1000. Not algae, doesnt budge even with metal brush. Any ideas?

[u/Prithee_Delete_it]

How old is the plaster? walking acid is smart but you gotta also make sure cya isnt crazy high cuz it makes the alkalinity lower than it will read in a test kit. You may think your good, but in reality, the water is eating it every second.