Mushroom removal

[Snaz]

If the foot is completely exposed you can probably use your fingernail and convince it to let go.

[Cal_stir]

If I inject it with a little vinegar do you think it would poison the tank?
I tried getting under it to pry it off but it starts to slime, there is about 5 large ones and they have a good hold on the rock.

[FitoPharmer]

If you want them gone for good and your live rock is not too hard I find a sharp razor blade can cut the live rock a millimeter or two below the foot with a little elbow grease.