plaster surface erosion

For the past half a year, the pH and total alkalinity were a little high, and calcium hardness has been a little low in my pool water. Not sure if the chemistry caused the issue, but I observed a few spots where the surface was eroded badly, and one spot even exposed the white plaster(?)...

I need help, Found Plaster condition in spa that was heavily Eroded. What's the fix? A Total Replaster? Also, after shutting off the pool pump, the pool filter was leaking from the bottom and Macy's Earth backflowed into the pool from the main drain and from the suction line? Can anyone tell me...