Go and get a test kit (not strips, they're inaccurate), then test your water. You may have water quality issues.
If it's an orange fromia or linkia, it's starving to death.
That tank is pretty small, and you have three aggressive species with the goby. The devil, sixline, and clowns are probably fighting, and the goby could have been caught in the middle. Is there any visible fin damage among the other three?
Don't assume that there's no aggression if you haven't seen it, fish often fight after lights out.
What kind of clowns? Most species get too big for a 28g.