Phosphates are the nutrients for Algae. They come from 150 different sources. If you eliminate phosphates you eliminate algae. Have you water test often for phosphates.  Order a phospate test kit and do it yourself.  If your salt chlorinator is having trouble maintaining chlorine levels check your phosphates first. It is the number one problem with keeping chlorine levels up in the salt pool industry.

Phosphate levels should be below 100 ppb.