Leviticus 13:36

Then the priest shall look on him: and, behold, if the scall be spread in the skin, the priest shall not seek for yellow hair; he is unclean.

Cross-reference

Leviticus 13:7 gives the same rule: if the scall spreads, the person is unclean — directly parallel to the ruling here.

Leviticus 14:39 applies the same re-examination after spread to mildew in houses — different context but same logic.