Id say cut it off and get rid of the bad one. dont take a chance.
maybe move the others to a new part of the tank if they are on the same rock. Either way, get rid of the bad one
I have had about 4 of my shrooms that have done that. They were doing GREAT one day then blammo, they just started to melt away over the course of a wek or two.
Good luck with yours.
Amy L. Cone