You can try to cut off the dead part, but what I've found to be successful is to pretty much make a new frag or two of very healthy tissue. As far away from the rtn location. This doesn't always help because I have had situations where not only the mother colony would rtn but also the new frags. But I've also had many successes with the new frags while the mother colony dies. Often something with your water is making the coral "feel funny" and if you can make water parameters as perfect as possible I've found this to stop my dying colonies from dying further. Hope this helps.