A white chocolate bar gets greasy gray-white streaks after warming and cooling, but still smells normal; what most likely happened?

Answer: Cocoa butter melted, moved to the surface, and set again as a gray film.

Milk proteins clumped into pale lumps as the bar warmed. — Milk proteins can clump in heated dairy, so the white color is a fair clue to notice. But a finished chocolate bar does not usually make gray streaks because its milk proteins bunch up. The streaks fit the fat phase moving and setting at the surface, which is why bloom is discussed with tempering and storage temperature.

Surface sugar dried into a rough, dry haze after moisture condensed. — Sugar bloom is a real chocolate problem, so this is a fair guess. It usually comes from moisture or condensation and feels dry or grainy, not slick. The bar here warmed, cooled, and looks greasy, which points away from dried surface sugar.

Cocoa butter melted, moved to the surface, and set again as a gray film. ✓ — Correct! In fat bloom, cocoa butter partly melts, migrates or separates, and recrystallizes into a gray-white layer on the surface. Unstable cocoa-butter crystals can set into a thin white film after the bar warms and cools. The same fat behavior gives chocolate its gloss and snap when tempering is controlled; poor storage turns it into a blemish.