He is probably hungry. The algae has died off for the most part and emeralds really only eat hair algae and diatoms/detritus.
I have 3 and I feed them the meaty sinking pellets. They will pick them up out of the sand or off the rocks. It's just something to keep them full so they don't so stuff like that. After all, they have to eat something. You can buy them at *****, and any LFS. They're pretty cheap. Make sure you get the small ones. (even the medium ones are huge.)
The shrimps LOVE them, FYI. My shrimps see them falling, and they do a little dance and come and eat the food. It's so funny to watch.