As others have noted, it's cyanobacteria. Your nitrate and phosphate levels are pretty good, so my guess is that it is primarily a water flow issue. Cyano does not need much in the way of dissolved nutrients to gain a foothold. In fact, levels that are fairly undetectable to hobbyist grade test kits can support cyano. But, as Crystal A noted, it does not like high flow.
For quite a while, I had a rather obnoxious cyano problem, despite great water parameters. A new powerhead cleared it up almost completely, in about a week. I now have only a small amount, and only in the one low flow area of the tank. I pretty much ignore it.