It's really hard to tell from the short description and not knowing more about the tank (a picture would help a ton) but it sounds like it could be Rapid Tissue Necrosis (not good), go and read some descriptions of RTN and look at some pics on line to see if it matches that. If it is RTN your best bet is clip off good tissue as a new frag. If you have to do this use clippers, don't snap birdsnet with your hands because the pressure from you fingers will damage the tissue.