Could be mouth rot...could be substrate, which could cause mouth rot. If you hold it gently by the base of the head where the jaws meet, while supporting the body so it's not just hanging there, and gently pry the mouth open with something that has a smooth rounded edge, you'll get a better look. Moth rot can look like sour milk chunks, and will form a lining on the outer edge of a snake's mouth as it progresses.

While I can't diagnos it for you, if it is mouth rot, you can treat it with a q-tip dipped in diluted peroxide or betadine twice a day until it clears up or you get to a vet.