I woudl be careful using flatworm exit if you have a large amount. A lot of them if not all contain a posion and if you kill too many at once you risk killing everything else in your tank...I've heard to happen to more than a few people.
I would try to siphon them with an airline hose, reduce feeding, increase flow if possible(they tend to gather in areas of low flow) and as a last resort flat worm exit. Some kinds of flatworms are actually quit harmless to corals.
There is also a type of nudibranch that eats flatworms, we call them blue velvet nudibranch but I"m not sure if that's the real name. They are black w/ neon blue and white lines. I've also seen six line wrasses, mandarins, and another type of green wrasse w/ two black spots on it's back(not sure of the name) eat flatworms.