I would take a vacuum cleaner with a soft bristled attachement and brush the mold off as much as you can without scuffing the surface. Most folks that deal with old leather for a living (museums, etc) just about crap their pants when you start talking about any leather preservative. The majority of these modern treatments are designed for new leather, to keep it soft and pliable. They are not good to use on old leather that you want to prevent from rotting away. Keep it in relatively low humidity and out of the sun, and that should keep them mold from coming back.