FW Dip is the perfect cure for those guys. Just make sure you bring PH and temp up in FW. Also use Iodine (Lugol's Solution preferred) and Flat Worm Exit. I do this to every colony/frag that I get before ever going into QT then FW dip again when they are ready to go into the display. The dip should not take more than 10 minutes with a lot of swirling and dunking to get those buggers off. Also, inspect for eggs. they are white and stringy.
The porblem is, when I saw him, we wasnt even near a colony. We was just cruzing along on some rock. Should I just wait till I see him again and squish him? Also, how destructive are they?
Now that you know they are in there, you have to watch very close to you zoanthids. You need to inspect them for eggs and for the nudi's.
If you want to be proactive and do the dips before you see a problem you can or just watch and wait untill you see zoanthid polyps with a problem.
I would NOT squish the nudi's with your bare hands. Use a rubber glove. The nudi's themselves are suppose to be as toxic as some of the zoanthids.
I spotted one on top of a zoa a couple minutes ago and picked him off with a steak knife. To my surprise, I found two more babies. Scraped them off too and down the toilet they went. I hope I don't have an infestation...