Albert Barnes Commentary Leviticus 13:20-21

"and the priest shall look; and, behold, if the appearance thereof be lower than the skin, and the hair thereof be turned white, then the priest shall pronounce him unclean: it is the plague of leprosy, it hath broken out in the boil. But if the priest look on it, and, behold, there be no white hairs therein, and it be not lower than the skin, but be dim; then the priest shall shut him up seven days:" — Leviticus 13:20-21 (ASV)

Lower than the skin – Rather, reaching below the outer layer of skin.