This could either be a food allergy, ringworm ( highly contagious to humans and other animals) fleas or acute moist dermatitis (aka hot spots). Since it is on the head of a puppy, I would think either allergy or ringworm. Allergy if left untreated can result in death and ringworm (actually related to athlete's foot and jock itch, not a worm but a fungi) is painful and unsightly.either one needs vet attention asap.