what is your plan? to kill it? if so, i'd just scratch it off with my algae scraper and then put some kalk paste over any leftover flesh still attached.
if you are removing it to give/trade/sell it, i'd use clippers and cut some of the rock off with the mushroom. you'll see that the live rock will just crumble off with the shroom, and the next guy will be able to glue it to something easily.