well how much rock did you buy with mushrooms on it? how much lr do you already have in your tank? if you are talking about cooking a 5-10lb rock and have quite a bit of lr in there.....it shouldn't be an issue
Preuss in Lansing will trade you an equal size piece of live rock for the piece you want to get rid of full of mushrooms. If you don't care about saving them, take out your swiss army knife, put the rock in a bucket and have at it!