Probably can't, as the finish is already disturbed, hence the rust.
Might try gently scrubbing with some ultra-fine brass wool & oil to remove as much rust as possible.
A cold blueing pen kit will cover the raw steel, but probably won't match the original finish.
Clean your holster & lube the interior with a food grade silicone spray. You can also apply a light coating of grease or vaseline to the slide during humid conditions.