Can we get a picture? This may help us see why...
Also, what are your parameters and what kind of lights do you have? What lights was he under when you bought him?
I'm still learning how to post pics on here here he is, I have an AI hydra 52. I don't know what he was under before, I had him shipped from live aquaria?
And turn off any power heads and wait a few minutes...a lot of times they prefer slow or no water movement to really throw out their feeding tentacles...then feed the frozen with a baster...