The day to day FC levels that we provide will prevent you from getting algae, but once you get it you have to follow our SLAM procedures to get rid of it. Once you get rid of it, then the normal FC levels will prevent it from coming back.
You've got to get aggressive and kill that algae off faster then it can reproduce. That means a SLAM. You'll want to lower pH first, then raise FC to 16 and keep it there by adding more bleach every few hours. And brush until your arms freeze up. Yes, it's a royal PITA, but it's still easier and cheaper now than it will be once the water starts warming up.
Stock up on bleach, beer, and motrin.
A tip: if you decide to buy stronger bleach, say at Home Depot or someplace else that sells pool stuff, watch out for old inventory. You might end up buying expensive salt water if it's been sitting all winter. Chlorine Date Code Decoder/Easy Way to Measure Strength. explains how to decipher things.