I would take the same guess that the baby blue tang swam into a hungry anemone. The little ones are especially naive when it comes to figuring out what to stay away from. The stress of an unfamiliar environment probably led
to an expensive meal for the nem.
Not to rub salt in the wound, but I realize baby blue tangs are practically irresistable, but they get huge fast and need a good deal of room to stretch their fins.
Close confines and an active swimmer are a recipe for disaster with an Anemone. Looks like a Sebae