The Lord addresses both Moses and Aaron. This is often the case in most sections of the Holiness Code (Leviticus 11:1; Leviticus 13:1; Leviticus 14:33; Leviticus 15:1). It was the duty of the priesthood to distinguish between clean and unclean (Leviticus 10:10). In this particular instance, the priests were to determine if skin abnormalities were possibly contagious and, by implication, whether or not an individual was clean.
1 The LORD spoke to Moses and Aaron, saying,