That is Cyanobacteria, I had an outbreak of it a while back, couldn't figure out what was causing it, it likes low flow, lots of nutrients and low oxygen levels.
I cut back feeding, cut back lighting, increased flow and added an airstone but it just got worse.
Finally used a product called Chemiclean by Boyyd, took about a week but it killed it all. Kept all my previously made changes and it hasn't returned since.