Keep a close eye on your goby; as sand-sifting stars, they consume micro critters out of the sandbed as their primary food source. Since your tank is not very mature, there may not be adequate food to keep it from starving. Not saying it will, just saying you need to keep a very close eye on it. If you don't see it eating additional foods you add, and it gets skinnier, then you need to rehome it.
"Listen to some of these guys talk, and it's like they were born from their momma's belly with a fishkeeping encyclopedia in one hand and an API kit in the other" (unrevealed).