vitiligo
it can get bigger with stress or friction, such as rubbing or scratching often
cant be born with it, its developed typically in adolescence
the white gets sunburnt real abd
ng things in passing like "oh yeah I got bullied for it when I was younger" or "I actually used to cover it up with clothes and makeup" are good ideas.
so it wouldn't be too unusual for a character to mention it every once in a while as well. Taking the time to be specific about where the vitiligo is and how "intense" it is would also help. Describing the vitiligo as patches or even patterned rather than spotted or speckled or even dappled