Charles Ellicott Commentary Leviticus 14:37

"and he shall look on the plague; and, behold, if the plague be in the walls of the house with hollow streaks, greenish or reddish, and the appearance thereof be lower than the wall;" — Leviticus 14:37 (ASV)

With hollow strakes, greenish or reddish. —If the house is really leprous, the priest on inspecting it will find in the walls the same three symptoms which are visible in the skin of leprous human beings: (1) hollow strakes, or, rather, deep cavities or depressions, which the ancient canons define as a depression deeper than the rest of the wall, being the same symptom as in man ; (2) a greenish or (3) a reddish spot, which were the second and third symptoms of leprosy in men and garments. (Comp. Leviticus 13:49.) According to the canons which obtained during the second Temple, the size of this discoloured spot on the wall had to be that of two beans.