Try some Hoppes 9 or even gun oil with a cloth and see how much of it will wipe off, soaking it in either for a bit might help loosen it up also. For any spots that do not just wipe off try a toothbrush or 0000 steel wool with some gun oil.
By the way, leaving a gun in a leather holster is just about the worst thing you can do for the finish, especialy in hot humid weather inside a car. Even stainless steel will rust in these circumstances. As Jason said if there are any spots where the bluing is gone you can touch it up with cold blue.