Looks like algae to me, but that can be just as attractive.
I have both moss and algae on the rocks in my falls. The algae that grows on them is thick and dark green and looks a lot like moss, but it's on the wettest rocks, so I assume it's algae. The moss is on all the ones near, but not in the water. There is moss on rocks in the water, but not below the water line.
Whatever it is, it will most likely die when you shut the falls down. But if it's moss, it should come back when you start them up again. Mine is all dead right now because my falls have been shut down for a while. My piping from the skimmer to the falls needs to be replaced and I cannot do it. Have a brother coming to visit who promised to help (he owns a watergardening business.)