well...if the conditions are stable and ideal for the coral, it will REGROW over the exposed area. however, if the tank conditions aren't ideal, the exposed area CAN spread.
Some solutions if you doubt your water conditions:
Cut the exposed area (as well as a few cm of good area) off. Then seal the freshly cut area with superglue (it's like a bandaid..kind of. it seals off the exposed tissue protecting it from infection.
if you don't want to cut the exposed area, you can just cover it with superglue (again, as well as a few cm's of neighboring healthy tissue).
Either way...whatever you decide, keep an eye on that spot. if it starts spreading, you'll need to take corrective action.
But all that aside, it's VERY normal (unfortunately) for there to be some tissue damage and exposed skeleton when you purchase corals from most LFS. The really good one's take care of their corals...but most just "house" them until someone buys it.
Now....quit piddlin' around, and post a picture!!!!
