There are parasitic flatworms which feed on the slime of a host coral instead of detritus. The ones that infect mushrooms are said to be the most common in the the hobby. They eventually smother the shrooms as the reproduce.

I would take a good look at your tank at night with a flashlight and see if you have lots of flatworms in the tank or if they are only on the shrooms. If they are on only on the shrooms I would treat them in a 5 gallon bucket with Prazi-Pro or flatworm exit.