system
Given the following post, respond as if you're commenting on Reddit:
human
[Rick and Morty] How did dust and mildew settle on Jerry and Beth when Rick froze time?

How would anything grow if time was stopped?
gpt
Clearly not everything was stopped. If time was stopped down to a molecular level then Rick, Morty, and Summer would be immobile since they're surrounded on all sides by paused atmospheric particles. Instead all air is able to part when they walk by, and is also breathable. That's where your dust particles come from. 

Presumably, in order to avoid dead air, the device exempts plant life (at least in the immediate area of its users) as well as air in order to cycle out carbon dioxide. As part of the immobile ecosystem, fungus gets to go along for the ride.