If you can find it, Lysol Concentrate seems to work quite well for killing off surface mold and bacteria. This product comes in a small bottle and is used diluted with water. Most likely sold where old fashioned products are found. If you have chlorhexidine on hand, it might also work to kill fungus and bacteria which are the usual problems.
I’ve had good results with the 50/50 white vinegar and water. IF (big if, lately) you have a sunny day, leave it in the sun for a few hours. Follow up with Leather Therapy conditioner as it has mild inhibitors in it. For a daily cleaner, Effax Leder Combi spray (https://www.effax.de/marktuebersicht.php?mk=4&p=629&s=%&g=&l=en) has a mild/mildew inhibitor as well.
I very well may be in the minority but I use CK shampoo on moldy tack. It is an antiseptic shampoo that treats rain rot and scratches and doesn’t leave the leather dry. I’ve had great results with it.
Leather Therapy and be grateful that you got mold…it means that the leather is still ‘alive’.
