I've read that polyp expulsion is a sign of high phosphates. If you can keep the loose polyp alive long enough (months), then supposedly the coral might secrete a new skeleton.

Had this happen once about 4 years ago with a brain in my seahorse tank & yes, the phosphates were through the roof. I kept the polyp alive for maybe 6 weeks after it let go but eventually it got injured & died.

Good luck.

Anthony