It's not really about the mice.
The mice probably couldn't give a rats about a little mould on their food bars. The problem is that mould spores get into the air distribution system of the ISS and spread everywhere. It becomes very hard to eliminate mould once it sets in because of the sheer amount of hiding space it has in a system like the ISS.
The old USSR/Russian MIR station suffered from mould problems through the later stages of it's existence. The ISS is dealing with it better because of regular cleaning and a requirement in design that nearly all spaces have to be accessible for said cleaning.
Biting the hand that feeds IT
