I have heard many times, or should I say I have read (on the internet) many times, that that mold on leather does no harm.
Okay, what do I know. But I just cannot believe it. That mold is living off something. It is getting its nutrients somewhere. Do you think it's livinng on thin air? No, I think it is digesting the leather, or something in the leather.
I suspect most of what the mold is metabolizing are the oils and waxes used to treat the leather. The structural component of leather is keratin, which is quite stable chemically and hard to metabolize.