Tribal Blenny Nipping at Corals

I've been having a very hard time with my zoanthids - I've pretty much decided they aren't going to make it. I used Coral Rx dip, taken them back to the store who put them in "hospital" for a week (where they improved marginally and had a fresh water dip). Yesterday I bought a Birds Nest coral and brought it home and it was doing GREAT, then I noticed the Blenny nipping at it. Where it's being nipped is turning white.

I have mushroom corals that are doing extremely well - - do Tribal Blennies nip at corals? I thought they were algae eaters. I thought as I saw it occasionally nip at the zoanthids that it was eating algae. There is no algae on the Birds Nest coral. Is there some kind of disease that kills corals almost literally overnight?

Anybody have any experience with that?

Thanks