I dipped the coral in CoralRx earlier this afternoon and saw about 7 little graying things fall off, but nothing happened to these maroon things. Which is the nudibranch?
Good call, valasaraptor and Hannah. I'd guess a nudibranch if it moves, tunicates (or some type of algae) if it stays still. If it's a nudibranch, that's a sea slug that will often eat corals.
Well, I ended up picking the maroon stuff off with tweezers and it appears to have been some type of algae. Anyway, the zoa has flourished since then - covering up the "bald" spot already.