As HighlandPete indicates this is fairly common on BMWs. I suggest you wire brush the outside of the hub. Then as Pete suggests, brush some anti-seize on the outside of the hub. If the inside of the hub is rusty like Pets's pix, wire brush and sand the rust a way, then apply some rust-oleum paint protection.
Do NOT put anti-seize on lug bolts. Only wire brush them. That is a whole other topic.