Those are flatworms. You'll want to solve this problem asap. Get some airline tubing and suck out as many of the little buggers as you can. I've also heard freshwater dips are successful.
Since this is a frag tank, and I don't normally ever suggest adding livestock as a solution, but many keep something like a 6 line wrasse in their frag tanks as they munch on these buggers naturally.
There is also "Flatworm Exit". I dont know much about it, only that it exists and that people use it effectively to treat acro eating flatworms...though these are only common brown flatworms...both can lead to the same result, in this case being so abundant they will starve your coral of light.