Feeding Mural/Sandsifter Goby

I recently bought what was identified to me by the LFS guy as a sandsifting
goby. After looking on the web, I've identified the goby as a mural goby
(maybe they go by the same name...and I feel "safer" identifying it as a
mural). Anyway, I don't see him eating much. He did sift a lot of sand
when I first got him but now (after about 3 weeks) he's not sifting much.
Is it possible that he's eaten everything out of the sandbed? I have a
30-gallon tank. Also, he seems to get exicted along with the other fish at
feeding time; but I never actually see him consume anything. I've offered
flakes, sinking fish food and frozen brine shrimp.

Thanks!

Maddie