First off, welcome to R2R!!
I don't think that a freshwater dip will take care of them, but an iodine based dip like Revive should kill them. Make sure you inspect the entire frag with a magnifying glass to search for eggs. Monti eating nudibranchs rely solely on montipora as a food source and should perish rather quickly if they don't have anything to eat, so you may want to consider placing the frag in a quarantine tank. This would give your the opportunity to keep a closer eye on it and also to get the nudis out of your DT. Another option for killing them is using Potassium Permanganate, but it's a very harsh chemical that is usually reserved for tanks filled with montipora. It often stresses the coral badly enough that something frag size might RTN... so just in case your research points you in that direction, I'd avoid that route.