TT #220 · Parshas Tazria · Aliyah 5 · Passuk 39
English: the priest shall examine them. If the spots are dull white, it is a harmless bohak that has appeared in the skin; he is clean.
Chabad.org — Chumash, English translation + Rashi for Vayikra 13:39
Torah Temimah cites Sifra (Toras Kohanim) for excluding bohak from impurity through spreading.
Sifra — Vayikra 13, Midrash Sefer
English: One might think that bohak is pure as a lesion in itself, but that it becomes impure if it spreads. The Sifra rejects this from “it has appeared—pure”: spreading does not make bohak impure.
Textual derivation
Even if a pale bohak spreads, that spread does not make it impure under the netek rules.