Cyano bacteria it is
Start with siphoning it up. Afterwards, reduce or turn off white lighting or all lights 3-5 days.
Add 1.5ml of liquid bacteria such as micro bacter 7 per 10 gallons for 5-7 days during the day.
At night, add 1ml of 3% hydrogen peroxide per 10 gallons at night
Clean filters and empty skimmer cup daily during the 5 days
After lights on, add snails such as nerite, margarita, trochus and nerite which will clean up any cyano remnants