[christyf5]

I have no idea what an umbellulifera sp is. Looks pretty cool. Oh there it is in google.

I'd say that the fact that it looks remarkably similar to your umbellulifera yet slightly different that it is likely a nudibranch (looks like it to me) or some sort of other predator that is specific to the coral. If I were you I'd yard it off of there right quick and get rid of it (keeping it in a separate tank will only prolong its death as I would expect that it only eats that type of coral).

Then again, some would also say that the critter is neater than the coral so you could leave it be, watch your coral get eaten and occasionally buy new ones to feed it.