dragon goby? Are you talking about the violet goby? Gobioides brousonnetti?
Or maybe the dragon wrasse? Novaculichthys taeniourus?
If you are talking about the dragon wrasse, it is a carnivore and will eat pods
and other crustasian in the tank as well as snails, small stars and worms, etc. They need their diet supplemented with meaty foods like mysid or small bits of fish.
From my understanding of the dragon goby, it is a brackish water fish. It can live in a full marine enviroment if acclimated correctly. It also needs plenty of supplemental foods to thrive.