I purchase a home in southern Texas in March. We have a in-ground (36x16) 18,000- to 20,000 gal. pool,White Plaster. I had a small crack in the bottom so we decided to have the whole pool re-plaster by a professional company.By April it was finished. The Pool guy we hired came out the day it was finished and said to fill it, and he would acid treat it once it was filled. when filled he poured in the acid and brushed the pool with wire pool brush, and had us brush it everyday for about 6 weeks. It is now August, we have a build up on the walls that looks like sand (rough). it started to turn green so our guy shocked it, but the green is only were that build up still remains. It comes off if i scrape it with a putty knife. but i do not want to damage the plaster. can you tell me what is going on?