Well.. on earth it took fungi some time to evolve the ability to break down wood. That's one of the reasons for coal and oil deposits. Fallen trees would not decay but simply stay around until they where buried and the material would naturally separate over millions of years.
It took roughly 30 years for a bacteria to be able to digest Nylon. (
https://en.wikipedia.org/wiki/Nylon-eating_bacteria)
What i'm trying to say is, Plastic eating Fungi are not completely impossible.
As long as a microorganism has both a use for the material, and a way to break it down you could argue that even metal eating germs are as likely as ammonia based space bugs