Turf algae. Isolate it in a fuge or KILL IT WITH FIRE!!!! If you need to see what it looks like because CUC don't keep it down you can always ask for photos in my tank, yes I have things that eat it, no I have only had it in my tank less than two months, yes it is in every rock in my 125, no you can not remove the roots from the rock to kill it. No my nitrates are very low and it can still suck up nutrients before my test kits can see it. I think you now know how I recommend doing things.... I will take aiptasia any day!