I have my doubts the Emp will eat the sponge. None of my angels have touched sponge I've tried. You could try helping those odds, try and drop mysis/whatever foods the angel normally eats onto the sponge to try and get him to nip at it.
When we say 'sponge' we are talking about a pretty wild category. In the wild, most angels tend to eat smaller sponges, hitchhiker type sponges, and a lot of tunicates (sponge relatives). Tree sponges probably represent something the angel has never tried to eat before. Some do take to eating them though...