White spots on plaster.

I am having trouble with white spots forming on the plaster of my pools. Particularly with the pools that use salt water & keep the pool heated. The chemical readings are maintained well within the standard levels. My guess is that calcium is forming on the walls during the heating process. My question, is there a known fix. Maybe a chemical additive. The only thing that I have found that works so far is to scrape or rub off the spots with a stone or sand paper. This is very time consuming & not fun.

Thanks in advance for the replies.