You can try feeding live brine shrimp to get them to eat, but being so small, I don't know if the goby would even be able to eat that.
I don't know about the green as opposed to the yellow, but I did it because the lady at the lfs
told me that one was eating. I've started asking if the fish are eating before I buy just as a boost, if it's eating, it may be adjusting better to captivity. You might actually ask them to feed the fish you want if they haven't already fed the fish. Petco does that for me now, if I go in early enough.
Of course, i didn't think to start doing that before I bought the Mandarin! But I have a lot of pods
and add some periodically to my tank, so my mandarin is still fat.