try wiping it with alcohol (dillute with water if you want) and have it aired before storing depende sa leather so try it on a hidden patch of the leather, i do this with leather jackets and suede before giving them out. kaya before storing leather bags in proper dust bags you need to somehow clean them first or do maintenance by using protectants every so often. tho sometimes ones the mold is there nakakadiri na sya gamitin so best is to just throw or give away after cleaning it a bit.
or take it to a bag place for clean up if brave ka naman try dabbing lemon juice duno if that works tho.