What kind of sleeper goby is it, specifically?
I had a golden-head/blue stripped for a while in a 55gal tank. He sifted a lot, and ate frozen food but almost every day developed a sunken bully. Once he ate, it filled back out & he was fine.
It wasn't until we upgraded our tank to a 75gal that he could go every other day (or every two days) without feedings. He never once developed a sunken stomach in the 75gal. I know liveaquaria says they are okay in a 50 gal
tank, but I definately wouldn't recommend it. My goby was much healthier and happier once he had more live sand.
Also, it was not uncommon for ours to hide at all. There were even times when the only sand he would sift was the sand to make a new hiding spot. If yours looks okay, I would say its normal. But, I will also mention, once we put him in a larger tank, he was out all the time.
That's just my personal experience, and I don't mean to lecture or anything, but I really did notice that much of a difference once our goby was in the larger tank. Unfortunately, though, our's perished due to an ick outbreak...
But I hope that helps, and I'm sure others will be over to offer some other advice.