It appears that nobody knows where the dan in dandruff comes from, but the second syllable probably derives from the Old English hreofla meaning leper. So perhaps back in the mists of time there was a particular flaky chap called Dan.
Apropos nothing at all, I feel duty bound to tell you that according the wonderful internet, there is a thing called leper juice. It’s the stuff in the lesions. ….. .
Incidentally, the World Health Organisation think leprosy’s got a bad reputation and prefer to call it Hansen’s Disease. This must be a great comfort to those dying of leprosy, but not to people called Hansen.