Albert Barnes Commentary Leviticus 13:51

"and he shall look on the plague on the seventh day: if the plague be spread in the garment, either in the warp, or in the woof, or in the skin, whatever service skin is used for; the plague is a fretting leprosy; it is unclean." — Leviticus 13:51 (ASV)

A fretting leprosy - that is, a malignant or corroding leprosy. What was the nature of the leprosy in clothing, which produced greenish or reddish spots, cannot be precisely determined. It was most likely destructive mildew, perhaps of more than one kind.