Newcomer to the forum here.
Does anyone have a clue what this substance is on my pools walls? It's green, hard, and gritty and seems to be imbedded into the plaster. I have tried scrubbing it off with a steel brush to no avail. Since the attached picture was taken, I have added water (low level in picture), shocked, brushed, vacuumed, cleaned filter until the water is clear with no new algae forming. This stuff refuses to come off no matter how much I scrub.