If the rust is light and only on the metal mount, it’s usually more of a cosmetic issue than an urgent problem, but it’s reasonable to clean it.

Safest first approach: use a mild baking soda paste (baking soda + a little water). Apply it only to the rusty metal area, let it sit briefly, then wipe it off and gently scrub with a soft toothbrush or cloth. Keep moisture minimal and away from the inside of the lens.

Another option is to dab a small amount of white vinegar onto the rust with a cotton swab, then wipe it away with a damp cloth. Because vinegar is acidic, use it sparingly and only on the affected metal.

Avoid sandpaper or steel wool unless absolutely necessary. They can create abrasive particles that may get into the lens or mount and cause more problems.

A rust remover like Evapo-Rust has been reported as safe around mixed materials, but for a tiny spot on a lens mount it may be more product than you need.

Work carefully, use very small amounts, and dry the area thoroughly afterward. If the rust is very minor and not affecting mounting or operation, leaving it alone is also a valid choice.