that is black brush algae. remove those rocks immediately and treat with a 100% bleach solution. that stuff will overrun your tank. it spreads spores that attach wherever they land and you'll have new tufts everywhere. It is said that only SAE's will eat it, but that's not entirely true. I googled about it and read that some people had to completely tear down and nuke their tanks and accesories just to get rid of this stuff. Some other people have also succesfully battled it by adding more/alot of CO2 and having no testable Phosphates. That stuff is bad, remove as much of it as you can, while you still can.