Are you now thinking there is cyanobacteria? If this is it, then it is organics that cause it, period. Light is obviously necessary, but it is organics-related first.
I have battled this a couple times, most recently last month in my 70g. I cleaned the canister filter and it almost went away just from that, but in a couple weeks it began returning and within a week was covering the floating plant roots. I did a thorough substrate vacuum over about 1/2 the tank where I could get to, and by thinning out some chain swords in the process; removed as much by hand from rooted plant leaves as I could. A couple major water changes too. I also reduced the fertilization (stopped it for a week) as in this case it was probably related; I had been doubling the nutrient supplementation in an effort to get my Frogbit growing better, but that was unsuccessful and it most likely contributed to the cyano.
This week it is gone, totally, not a scrap of cyano in the tank.