Suposedly, he has vitiligo, which is basically a lack of melanin which provides skin color and UV protection. It started on his hand, which explains the one white glove he used to wear. My guess is that it started to spread to more obvious areas and he started receiving skin treatment (bleaching) to cover it. In his interviews, he's said that he's never bleached his skin, which to me shows he's in denial. Most people with vitiligo learn to live with it. Jackson has, or has had lots of money, so i he can pay to have it treated and act like it never happened, although there's been an obvious change in his skin color.