So, what now? Bearing grease inside the axle tube? That's just weird. The axle seals are known for going out and letting the gear oil make it into your knuckles, but I can't see how any significant amount of knuckle grease would go in the other direction, not and stay solid grease.
I'd chalk that up to a weirdness and move on. Hit it with some degreaser if you're feeling diligent, but I'd probably scrape up what I could with my hand and ignore the rest.