Wow, I just reread my post and realized it was quite possibly the worst response to a question I have ever given. My internet can be pretty slow here and I only saw the first pic is all I can think of.
Sorry about that. Is the coral still in decline? Euphyllia sp. normally don't harm each other. The tang may be the culprit. Feeding him Nori sheets usually bstops them nipping on corals. They have been known to do this if there is something lacking in their diet. They eat meaty foods but this isn't their preference.