You can use a clean terry cloth towel and a 50/50 mixture of 91% pure isopropyl alcohol (from any drug store), and distilled water.
The mirror is a "first surface" mirror, meaning the coating is on the outside of the glass (as opposed to your bathroom mirror), so it's very delcate. And if it's just dusty, you might try just the terry cloth towel and very gently wiping it. It really shouldn't be that dusty, because it's always "looking down" so dust doen't usually adhere to it.
Let me know if that helps, and thanks for using JustAnswer!
Sure, that would work, too, as would any lens cleaning fluid for a fine camera.