וְרָאָ֨ה הַכֹּהֵ֜ן אַחֲרֵ֣י ׀ הֻכַּבֵּ֣ס אֶת־הַנֶּ֗גַע וְ֠הִנֵּה לֹֽא־הָפַ֨ךְ הַנֶּ֤גַע אֶת־עֵינוֹ֙ וְהַנֶּ֣גַע לֹֽא־פָשָׂ֔ה טָמֵ֣א ה֔וּא בָּאֵ֖שׁ תִּשְׂרְפֶ֑נּוּ פְּחֶ֣תֶת הִ֔וא בְּקָרַחְתּ֖וֹ א֥וֹ בְגַבַּחְתּֽוֹ׃
Y el sacerdote mirará después que la plaga fuere lavada; y si pareciere que la plaga no ha mudado su aspecto, bien que no haya cundido la plaga, inmunda es; la quemarás al fuego; corrosión es penetrante, esté lo raído en la haz ó en el revés de aquella cosa.
Rashi on Leviticus
הֻכַּבֵּס אחרי AFTER IT IS WASHED — The verbal form is an expression of something done (a passive infinitive).
Rashbam on Leviticus
פחתת, the word describes a kind of inflammation which eats away at the affected area.
Tur HaArokh
פחתת הוא, “it is an ingrained affliction.” The word is derived from פוחת, something which gradually diminishes, becomes progressively worth less, until eventually it has ceased to have value; in this case the meaning it that the damage will spread until it encompasses the entire fabric.