Folks used potatoes in humidors for decades.
So far as mold goes, first imagine people who had nearly nothing. Then imagine how dear tobacco was, and how much everyone loved it. It was literally a societal standard. Imagine how ingenious it was to use a potato to keep tobacco fresh. Then imagine that these same people who were smart enough to use a potato were more than intelligent and motivated enough to change the potato at regular intervals.
Furthermore, a potato will literally last for months, and easily a year, when kept in a high humidity (which it would create), enclosed, cool environment. In a tobacco jar in a cool place, it'd last well over a month, even if only a small piece was used.
There's your potato/tobacco lesson for today. Don't take my word for it, get your google-fu on. It's pretty interesting stuff if you're bored enough.
I bet some of the old timers here can remember when their grandpa had a piece of potato in their tobacco.
Don't think slice of potato. They'd cut off an end and set the skin side down on top of the tobacco. That or they'd use the little tiny potatoes that you get when you grow your own.