Do you have a picture that's no so blue?
It's hard to see what he looks like, but here's where my mind is jumping to. You said it ate really well. What did you feed it and how often are you doing that? I feed my carpet anemone maybe once per week and my long tentacle about half as often. Each one gets half of a silverside at a time max. they actually should get most of their nutrition from their zooxanthellae like a coral and shouldn't need to be fed often.
From what I can see, the anemone just looks like it shrunk, which they do from time to time to change their water and expel waste. The mouth seems closed and the foot looks OK, so I wouldn't be too worried at this point. give it a day or two and see what it looks like. Don't try and move it from where it's attached.
The cause of a lot of anemone troubles goes back to just a few things: physical trauma (sometimes from aggressive clowns or during collection / shipping) , water quality, insufficient light, and overfeeding.
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