Tweezers. They are pretty hardy ricordias. He may be ****ed off for a day but gently pull him off and he will be fine. I've pulled them off rocks before.
That sounded like the opening line to a joke....How do you get a Ricordea off a Sea Urchin? Anyone, anyone?? I agree with physically (but gently) removing him. Stuart at Mile High Aquatics removed some off some rocks recently when I was down there and also said they are very hardy and resiliant. That's his bread and butter (GREAT corals by the way - quick plug for him :wave so I trust he knows what he's doing. Mine has opened up since then nicely.
+1 when I was there he picked up a few, chopped them in half with scissors, threw them back into the tank and said they'll be completely healed in a couple days