I would NOT use a razor blade. Anemone tissue is very delicate, and you would likely slice him up given he is on an uneven rock. Removing bta from rocks is different from flat glass/acrylic.
Personally, i frequently use my fingertips to gently feel around their base, slide fingertips and short nails under, and urge them to release from the rock. The first times you do this will take a while, potentially an hour or more. But after lots of experience, it can be seconds/minutes to remove an anemone safely.
But thats just what i would do.