Yeah, with that high CYA, I suspect your Cl levels have, in the past, dipped low enough to allow that algae. I think you're in the process of eliminating it with just chlorine.

Algaecides get mixed reviews on the forum. I've never used them, others seem to have very good results. Your conditions might be ideal for the use of an algaecide as a preventative.....might not be a bad idea.