What's this stuff?

[William Sharkspeare]

On the rocks in a couple spots as well. It brushes off really easily with a toothbrush. Hard to pick up with a gravel vac. Had to take the vac off and just use the tubing taking the sand with it.
NO3 2.5
PO4 .05

 

[EnterName]

While dinoflagellates are usually brown we have seen some green variety from time to time. A compound microscope would settle this. There also is a chance of it being green cyanobacteria (rare in reef tanks), or Euglenids (even rarer).