Michael Jackson has a skin disease called Vitiligo. This destroys the pigment of the skin and turns it very pale white. It was perhaps hereditary - his grandfather had it. He's been through a lot. As a child, he was abused by his father who called him ugly and beat him. The trauma of his childhood triggered it, and it became noticable when he was an old teenager. In early 1984, he was burned during a Pepsi Commercial, and this worsed vitiligo and also led him to suffer from Lupus as well. This did the entire opposite of vitiligo and gave him red rashes on his cheeks. He put on lots of makeup. During the Victory Tour his skin did not look natural, because there was so much makeup and when he was sweaty it would come off and show the white patches of skin.
Around 1987, his patches got so extensive he gradually used paler makeup.By the Dangerous era, he used extensive makeup, and his skin was variably yellow or white. During the HIStory era, he looked the same but even paler.
Perhaps he did bleach his skin, but it was because of vitiligo. Stage outfits have been found with skin bleach. In fact, this is what his doctor confirmed, but he probably used it because he had lupus, which made his skin more pigmented during the Thriller era. It's not possible for him to do this without vitiligo as his skin would appear very rough and nothing like what it does now. But Michael is not alone, many others have bleached their skin due to this devastating disease.
Because of Vitiligo, Michael cannot be in the sun and carries an umbrella on sunny days. Even though he may have bleached it, the pigment cells still return occasionally and he wears the mask to conceal them. His trademark glove, as well as his long sleeve costumes for most of his live concerts are also there to conceal the vitiligo. Vitiligo can also affect your personality. He was very emotional when he talked about it and he became more reclusive and depressed. He became even more depressed due to his traumatic childhood, and the accusations of child molestation, from people who betrayed him just to get money. He's been betrayed by many people, but many still love him!
As in the way Michael sang in Ben:
"I don't listen to a word they say!!!"