I think you should transfer the goby to a hospital tank where you can spot-feed it and possibly begin treatments. If you post a short video,
@Jay Hemdal may be able to diagnose the problem, which may well be flukes. Gobies should be fat, glaring at you with anger, and possibly plotting world domination, instead of sitting listlessly at the bottom.
I haven't had a problem with sand-sifting gobies being unable to sift "special grade" sand or crushed coral, for that matter. My golden head goby enjoys picking up larger pieces and tumbles them in its mouth, and then spits them out. It then goes back to its regular job, trailing sand behind it like a chain smoker.
Keep in mind that these gobies need to be fed frequently. Once a day just isn't enough.