Is your axie a wildtype axie or leucistic (white with darkish eyes) or type of albino (golden/white with reddish eyes).
The dark under belly may just be colouring. My sister has 5 dark wildtypes/5 leucistics. The wildtypes all vary in darkness, dark browns to blacks with greyish sides or grey/dark underbelly.
How old is your axie and do you have an airstone and filter in your tank. If you have enough oygen in tank axies gills will be smaller. Are they healthy looking gills tho they're smaller, ie; all the gill filaments/hairs are still on the gill stubs or do the gill stubs look a bit bare as well as small)?
An axie will sometimes sit with it's mouth open a bit. Have you tried live worms, someone tried live crickets once (not sure how they're fed though), or even try a piece of lean red meat (cut all fatty bits of) cut in inch long thin strips and jiggle it in front of him; or axie pellets or mealworms (cut off the head tho).
What is the ingredients in the special green food?
Have you tried frozen bloodworm cube?