With all due respect to Dwayne - I would use plain old white or cider vinegar first to clean and kill the spores, then clean with a good saddle soap per the instructions ( I like the PH balanced liquid saddle soap for cleaning). This is the way all the old timers I learned from cleaned off mildew/mold and after 50 years at doing leather repairs on everything from wallets to saddles, it's IMO the best method and if the leather is not overly damaged will bring ti back up to snuff....
After cleaning then use a good conditioner such as Lexol - use it sparingly - over conditioning is just as bad if not worse than under conditioning leather......