Animal skins are populated as albumin-containing material to leather of microorganisms already during her/its/their processing. In general, the direct damages, that evoke mold on leather, are not serious. So, molds build natural or during the processing of too bulky fats from. The set free fatty acids form a fine white coat at the surface. The availability of the water is with leather bigger than for example with wood. (Siehe here under point 7.4.3. to aw-Wert, therefore each material doesn't mold with the same atmospheric humidity. Also bacterias cause discolorations and mold spots.