The diff oil should never get into the bearings. It's grey because it's old and has picked up metal during use. Bubbles, had you just driven it or something?
The hub should be full of grease, the swivel then full of EP90 or one-shot hub grease. If the locknuts are rusty then the bearing seal has let water in, the bearings NEED cleaning ASAP before they rust and the swivel may also need a change of oil. Likely as not the stubb alxe will have corroded where the outer hub seal runs and the seal will also be damaged.