Mine did the same thing. Pestered the bubble tip til it moved all over the place and then looked like it would die. REMOVE it if it reappears and I would look under that rock near the anemone cuz it's probably still there! My anemone has since recovered so there is hope if you get rid of the crab. Even moving the crab to another area of the tank did not work. I tink they are best hosting in large carpet anemones.
Good luck!
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