I added this same fish (white watchman goby) to my 20 long, saltwater Molly tank, and here is my experience:
I feed the mollys flake, the goby eats flake
I feed the mollys pellet, the goby eats pellet
I feed the mollys Formula2, the goby eats it
Mysis shrimp, the goby eats it
and the kicker...
the molly had babies, and you guessed it, the goby ate them.
The more variety, the better, I think to have a happy healthy fish. Though I think fish fry are not a diet requirement, I have to admit I was okay with that fact. If you've had mollys, you'll understand that callous remark.