Poor hermit crab. You can just pull or slice it off the crab with no problem. I used to be squeamish and try to be gentle because I worried that I would hurt the xenia, but I don't believe that is possible. I'm brutal to it and it still comes back.
You can pull, peel, or cut it off. Peel it off if the rock is flat. This is the best way to keep it from comming bact at the same place. You can cut it off. This tramatize the Xenia more but it will leave tissue that will growth into new Xenia stalk.