One has to wonder what happens if brown mold gets dropped into a volcano.
There has to be some state at which brown mold goes dormant or otherwise stops growing. Otherwise you have entire deserts blanketed with brown mold in an afternoon and active volcanoes clogged with the stuff like very large magma-filled zits.
It should also be determined what level of mundane cold will kill it. Alchemists with alcohol and mercury thermometers in cold climes would have samples exposed and see at what temperature it dies.
Probably the best solution for the "brown mold covers the planet" scenario is to make it so that brown mold is not immune to its own cold, so once a colony reaches a certain size, it freezes itself to death.
That's probably why it's stated as coming in patches of 5' diameter, because if it gets too large it starves/freezes itself to death.
Also, Deserts get pretty cold at night, so that would kill it or at least slow its spread.