It sounds like they were stressed and or not under lighting for a long time and expelled their zooxanthellae. This makes it much more difficult for them to feed themselves until they are reinfected with some more zoox. The following statement is going to sound funny and against the grain.....make sure they are in a tank with some measurable Nitrates and dissolved organics so they have some food until they get more zoox.
BTW....this happened to me once in one of my nano tanks. I was on a business trip for 3 weeks and got a call that my lighting burned out like the first or second day I was gone. I returned to find that all mushrooms on the far side of the tank were white like you are describing. (The mushrooms on the left side of the tank could get a little ambient sunlight but the ones on the right couldn't due to the rock placement). I would say that 95% of them survived and recolored up.
Assuming that yours are white due to lighting issues, don't light shock them. Put them low in the tank and slowly move them up. If any of them expel their mesenterial filaments also (strings that will come out of the mouth in the center), cut that particular mushroom off the rock and dispose of it.