I have a shroom rock and a chunk of star polyps that were saved from being tossed because they didn't look right to their former owners. They are now thriving. Unless you see mass tissue disintagration or the hermits are feasting on something I would give it some time and see if it comes back.
Anemones are an exception, when they start dying off they get really nasty and need to be takin out immediatly, better yet don't get them in the first place.
Over the years I have had a few
toadstool leathers get an infection that causes
tissue necrosis through the stalk, destroying it from the inside out. By the time it shows up as a hole in the cap most of the damage is done. The best thing to do in that case is cut the stem off just above the anchor point and dip it in iodine solution to try and halt the pathogen. You can aalso cut the solid pieces of the cap away from the dead stem and dip them then attachthem to a rock. Keeping them in a bowl or shallow tray to protect them is a good idea. Hopefully they will reattach and you might save the coral by fragging it. I have a couple viable frags from the toad I lost after switching tanks at thanksgiving.
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I dont advocate holding marine creatures for ransom but......since you asked
Doug, plank owner
http://hellreef.homestead.com/index.html 
