Did you receive the coral this way or is this something you noticed after the frag has been in your tank for a while?
If the frag is new, it's possible it got hurt in the fragging process or during transport.
If it has been sitting in your tank for a while I would double check that there aren't any parasites or fish that are picking on the coral. If the coral had other issues I would expect tissue to recede evenly on all sides, but maybe it's possible to start on one side, I'm not sure.