If its on a solid rock I'm not sure you can. Hopefully if its not getting what it needs it will drop on its own. Not sure how well they do if you try to shave them off. :-(
I use a pair of wire cutters and snip off the rock around the shroom. You can cut the foot of the shrooms if they are attached to each other with a sharp blade or scissors.
They do attach almost exactly like nems. But its almost impossable to get them to let go if they don't want to. I think they are pretty hardy in regards to cutting them off. But not sure enough to recomend it.
They don't like metal.
If you stick slowly a screwdriver under the foot they will let go slowly from the rock.
Btw this also would work with Nems of course.
Well I'll try the metal thing since it's a 50/50 shot. However I don't think I'll be able to reach my arm to the bottom of the tank UNDER the chalice and cut the rock. [emoji53]