Was the anemone just added and stressed? If not, I would say stealing food. I had a Peppermint Shrimp go after a newly introduced, stressed from shipping anemone, but I would not expect that behavior from a Fire Shrimp. Likely just scavenging.
Is there a chance he might stress out the anemone? The shrimp wan introduced about 3 months after the anemone. The anemone had picked a spot in the tank. Since the introduction of the shrimp the anemone has been on the move. I have also soticed that there are times when the shrimp messes with the anemone and the anemone empties it’s stomach. Is this something I should be concerned with?