Hello, The skin turns white for the same reason that your feet or hands will turn white when you take a bath or have them in water for a long time. The skin get 'water-logged'. read more
The material from a band-aid keeps air from properly getting in and out of that location on the skin. Consequently, moisture is trapped in the skin in that area and the whiteness that you see is moisture buildup in the epidermis(outermost layer). It's like when you have your hands in dishwater too long. read more