It sounds to me that your new fish is shy at being in a new home. Also with no other fish in the tank, it probably feels very vulnerable. Any chance to add some small fish, so that it doesnt feel so all alone? I do not know how long you have had your tank set up or if its cycled yet or not, so its hard to suggest more fish at the moment. Other than to say, it may feel more secure with other fish in the tank.. and that it will get used to its new home in time.
Even tho its not eating the food when it floats, if you leave some in the tank, eventually the little guy will eat off the bottom, and this way get some food.
Do you have any plant cover for it? DG do like to have some floating plants to hide in at the surface and adding some plants may give it some security it is looking for.
Can you give us some more information on your tank? How long its been set up? What water tests have you had done? What is your water change routine if it is not cycled yet? All this can help with future questions, that we may be able to help you with.

Good luck with your lil guy!

~ kate