Do you have any other mushroom corals? If so, do they all look ok? There are plenty of people who...for whatever reason...just can't keep mushrooms. I couldn't keep them in my 40g to save my life. Has to do with the water being too clean...something missing from the water according to some of the "experts" on RC.
But...let's avoid that path. What are your parameters? Where is it located in the tank? Do you have a picture? Are you running carbon (to prevent coral-slime wars?)