Definitely not regrind. Regrind occurs in splotches, not dots, and can only lose color, not gain it. Looks like mold to me too, or maybe an old ink spot if you're lucky. I'd give her a double bleaching -- a couple swabs with the liquid kind to kill any mold, if present, and then the UV baked potato kind to fade the appearance of the dot. Boiling would work to kill the mold too, but her neon-colored hair might bleed, so keep it out of the pot if you go that route.