“But stuff has been growing on the outside of the ISS...”
No, I don’t think so. A few microorganisms were able to survive for a while outside the capsule, but only in a state of hibernation, so they weren’t growing.
Several studies have been published in the last two years concerning microrobial survival in interplanetary conditions. ex:
The third study was conducted on organisms that live on rocks. These bacteria were kept in the EuTEF test facility for over 1.5 years. Researchers wanted to know whether space rocks could transport life from one planet to another, a theory known as lithopanspermia. The team selected organisms that are known to survive tough conditions on Earth. The duration of the study was short. However, researchers believe that the experiment results provide evidence that hardy organisms could survive in space for millions of years before landing on a planet.