Looks like the first signs of mites (little scabs around jaw/shoulder area). Easily treated. Get some puppy/kitten revolution from a vet, and put 2 drops directly on the skin on an area they can't easily reach (behind the neck or shoulderblades) and let it dry completely before letting the rat go back to their cage. Treat ALL rats you have even if they dont' have the scabs yet, they probably still have the mites too if they live in the same home you do, they are really easily transferred. It's just effecting your one rat more than the others at the moment.