וְרָאָ֨ה הַכֹּהֵ֜ן אַחֲרֵ֣י ׀ הֻכַּבֵּ֣ס אֶת־הַנֶּ֗גַע וְ֠הִנֵּה לֹֽא־הָפַ֨ךְ הַנֶּ֤גַע אֶת־עֵינוֹ֙ וְהַנֶּ֣גַע לֹֽא־פָשָׂ֔ה טָמֵ֣א ה֔וּא בָּאֵ֖שׁ תִּשְׂרְפֶ֑נּוּ פְּחֶ֣תֶת הִ֔וא בְּקָרַחְתּ֖וֹ א֥וֹ בְגַבַּחְתּֽוֹ׃
And the priest shall look, after that the plague is washed; and, behold, if the plague have not changed its colour, and the plague be not spread, it is unclean; thou shalt burn it in the fire; it is a fret, whether the bareness be within or without.
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.