Cyano. Flow didn't help mine. chemipure (?? chemiclean??) made it go away temporarily. Good husbandry helped. Less food, more water changes, stirred the sand a couple of times a week to get rid of it.
I had a spot pop up a few weeks back. I did my regular water change, and used a turkey baster to brush it off the substrate. I added another powerhead to just add some turbulence in that area. haven't seen it since. It was in the corner of the tank that does not get good flow.