It’s Cyanobacteria, or Blue Green Algae (BGA). Actually not an algae but a bacteria that photosynthesis’s I believe?
Anyhow, it’s kind of a beast to get rid of. Usually the only way to completely kill it is with a fish medicine called erythromycin (Maracyn). I used Fritz Slime Out on mine and it took care of it super fast.
I haven’t figured out what causes it yet. I eradicated it a few months ago but I’m starting to see it come back in my tank… sigh.