my dehumidifier broke, so all the stuff in my garage molded.
i took a old pair of flea market leather chaps (UNIQ brand) that were lightly molded, and washed them in the driveway, with dish detergent and a scrub brush.
got all the white off, rinsed them off with the hose.
use leather cleaner not general household cleaners.
You can use saddle soap to clean and then treat with a leather conditioner to restore oils that were washed away any of these products should be available at a tractor supply or similar store.
Take the mildewed items outside to clean to prevent spreading the mildew spores.
Mix a solution of 1 cup of denatured alcohol (rubbing alcohol) to one cup water. Soak a soft white cloth in the solution and wipe away the mildew.
Allow the leather to air dry away from the sun or direct heat. If the item is leather shoes or a handbag, polish with a good quality leather polish or cream.
Tips:
If the leather is heavily mildewed, it may need to be cleaned with a solution of saddle soap and water. Use a soft cloth and work the soap in well. Rinse with a cloth dipped in clean water. Allow to air dry.
If the mildewed leather is on shoes, after they are cleaned stuff them with soft paper - like tissue paper - before allowing to dry. This will help them hold their shape
Use Mink oil on them after they have dried. It will soften and preserve them for many years. I just posted this same info on the boots thread. Any good leather dressing will work but I've received the best results with the Mink oil paste. I have a leather jacket that I treat this way that is over 25 years old and still like new.