Brad,
It almost looks like some type of caulerpa, but probably isn't.
If this was in my tank, I'd turn off the powerheads while picking it off the rock with tweezers. Having no water flow while picking the stuff off will prevent bits that escape the tweezers from floating off somewhere only to begin growing somewhere else. The bits that do come loose from the tweezers can then be picked up as long as you observe where they fall.
Of course, this is the PITA method of removal
To even make a dent in the stuff, you will probably have to go at it for as long as you can stand it every day for a week or more, depending on how bad the infestation is. Then, as it grows back, as it inevitably will, take another round at it. You may not remove it all, but at least you may be able to get it to a manageable state.
Wish I had a suggestion that made it go away without all that effort. Whoever said reefkeeping was a passive endeavor wasn't very well informed, ime
