This refers to the material with mildew on it. This can be translated in active form. AT: "the contaminated material that they scraped off" (See: [[rc://en/ta/man/translate/figs-activepassive]])
A place that is unfit for people to occupy or to be used for God's purposes is spoken of as if it were physically unclean. (See: [[rc://en/ta/man/translate/figs-metaphor]])