I agree that you should try to shave the rock instead of cutting the xenia. If you do have to cut it, what I suggest is to run a piece of thread or fishing line through the stalk first with a sewing needle. This way, when you cut it and its getting all upset, you have something to use to anchor it someplace new. If your just using it as fish food, don't worry about the thread.
Also, when it's upset, it's gonna slime up and stink! If you can get the rock out ot the tank, do that, cause you don't want that slime all over the place. I'm no expert, but that can't be good for your other inhabitants.