vitiligo:

An autoimmune skin disorder caused by attack of the immune system on the pigment-producing cells within the skin. The loss of the cells responsible for skin color result in milky white patches on the skin surface. 

disorders characterized by large patches of skin, which lose all color and appear milky white. Some cases can be treated with permanent cosmetic implantation of skin color. 

an acquired skin disease characterized by patches of unpigmented skin (often surrounded by a heavily pigmented border)