The most common definition for rust is iron oxide, but can also mean corrosion on other metals other than iron. As a verb, it can also mean to degenerate with the passage of time with no mention specifically to iron. As an adjective, it can mean inept and slow through lack of practice or old age.
All 3 of those alternate definitions could apply to "rusting copper" although oxidizing, corroding, or more generically deteriorating copper probably would have been better.