I've used lemon juice, Aiptasia X, Joe's Juice, and I had an Aiptasia eating file fish. If its just localized, I'd try Aiptasia X or Joe's Juice. The key is to have your pumps off for about 15 minutes while you treat. I've also looked into berghia nudibranchs but don't have any experience with them. Might want to consider some Peppermint Shrimp as a natural defense as well.