If you have time, what I often do is place other rocks up against the coral you are tring to move until they start to colonize the new rock and then using a scalpel, cut off the ramaining parts of the coral attached to the old rock. With the nepthia, if you have to cut them to move it, make sure it is a very clean cut since they are very suscpetible to bacterial infections at the site of incision which can quickly spread.