If the Master was in one of his moods, or if this were the Doctor, such cheekiness would force him to storm over, his face darkening in rage, and strike his captive for her insolence. As it is he simply smiles, tolerant for now, and answers his own question.

"Mildew. Oh, it was faint, probably undetectable by human standards, but they couldn't keep a thing from me. Imagine it. Centuries of hidebound tradition, insufferable pomp and pretense. A planet so chained by its own sense of self-importance that it couldn't learn to save itself if it tried." The Master chuckles, equal parts disparaging and somber, and perhaps it's clear now he's no longer simply talking of Earth.

"Well." He turns to look back at River, brightly, without a trace of moodiness now. "All gone now. Time to get down to business." He approaches her with a malevolent glint in his eye.