Cyanobacteria. Lives and thrives off of nutrient issues and low flow in aquariums. Same advice as before, cut back on feeding heavily. Add another powerhead maybe to help things out. You should have 50x turnover of the water by the size of your aquarium.
I'm wouldn't call myself one of the most knowledgable on here but your tank should go through the brown algae, then a slime algae cycle and then you'll see green hair algae. I waited until then to add my flame angel (about 6 months after set up) and he is doing great. I know he grazes so you may not have enough of what he needs to graze all day.
The slime algae is always a horrible process to go through. You can cut back on your light but its just a stage that you have to go through. Physical removal is the only thing that worked for me until it "just went away." It is going to cover everything but your tank will get established/balanced and it will eventually clear up. They have chemicals to help but I am not a fan of the slime algae chemicals.