I'm assuming that it hanging on to a piece of live rock.
Sometimes on the first attempt if you get a finger nail under the very edge, you can peel it off, slowly and very carefully.
I've used a small powerhead with the flow directed at the edge iof it to get it to lift a little so I could get a fingernail under it.
Also I've read that a ice cube right on the edge of the foot will get it to lift a little.
Don't use anything sharp, they tear very easy.