So if I have them, how do I get rid of them?
QT all new corals first, sorry dips are not enough (emphasis added). Second, as Joel suggested, the easiest treatment method would be to remove all your existing montipora corals from your reef and DO NOT add any new monitpora corals for a long time....up to 4 to 6 months. If your remove the pest's food source (ie, the montipora corals), the montipora eating nudibranchs will starve to death after about two to three months or so.
If you want to treat your existing corals, you will need to first set up a separate quarantine tank then you will have to treat and kill all the remaining nudibranchs and their eggs. Iodine dips – (ex. lugols iodine, Tropic Marin pro-coral cure, Seachem reef dip) can kill the adult nudibranchs, but will not harm the their eggs, which would need to be manually removed or scrapped off the coral. Another treatment option for chemical baths is Levamisole, a drug created to treat worm infestations in animals, which is available by prescription only.
I will be giving a Coral Pest and Quarantine Presentation on Feb 17th to the members of the Cincinnati Reefkeepers Society. If you would like to learn more about SPS pests and how to identify, treat and quarantine new arrivals, feel free to attend the meeting.