It's a type of aeolid nudibranch definitely.All of them are specialize predators of a specific coral or anemone.For example,berghias feed only on aiptasia,zoa eating nudibranches feed only zoas.
oh no im sorry to hear this. Nudis are a nightmare. Go through that zoa frag you have and see ifs theres anymore on it. Chances are if theres one that big theres alot more. Look for little egg clusters too. Pick off all the ones you see. Do an iodine did. Then just keep repeating every week or so till there all gone. What you can do too is just take the zoas out of the tank and theyll starve. Keep the zoas about for about a month.
oh great....
it's the only one I've seen so far, but I'll take a better look today. I only have the one zoa frag, so hopefully I can get a handle on it quickly if there are any more....strange, when I found it, it was nowhere near the zoa....