Vinegar will kill the mold spores,,,
Make a 50/50 mix of vinegar and water,,,
Use a clean sponge and scrub the leather real well.
Wipe the holsters dry and then let them air dry completely,,,
Use a light coat of neatsfoot oil to condition the leather.
I prefer and use pure neatsfoot oil,,,
Any commercial leather conditioner will work,,,
The trick is to not use too much of whatever you choose.
But the vinegar will definitely kill the mold spores,,,
Just make sure you get it all with the sponge.
Aarond