Cyanobacteria. Some people call this "blue-green algae." For obvious reasons, as you're learning. This stuff is not exactly like Schizothrax (which is the red "cyano" in low-flow areas of everyone's tanks); pounding it with current does not always eliminate it.

When I see this in my tank, I do 2 things:
1. Get out a turkey baster and pull this stuff off (it's easier to get out than red slime "cyanobacteria," because it stays in sheets better);
2. Increase my KW drip. This is my continuous soapbox topic, as you've probably noticed: If coralline algaes are covering your rockwork, it is very hard for anything else to grow there.

Look back over the last 3 months on this site for algae, KW, and calcium additions. There's more detail....

Thanks,
Dean