Zoa eating nudi, which is bad, bad, news. My whole tank ended up in the dumpster because of those things. If you only have a few zoa's in your tank, and you can remove them, you should be able to get rid of them. I would suggest setting up a small tank and putting all of your zoas into it, and treating the zoa's separately, and letting the nudi's die off in your display tank for 6+ weeks.