Flatworms can quickly take over a tank, and dipping your corals won't help because they are still on the rocks and glass. There are chemicals you can add to kill them, but I don't think they are reef safe. Your best bet is to pick up a sixline wrasse, they eat them. Mandarins also eat them, but if you don't get a mandarin that also eats frozen food it will just starve eventually. I'd go with the sixline if I were you.