"This of course raises the possibility that the soon-to-active alien slime colonies of 67P/Churyumov-Gerasimenko might find much of our planet to be prime real estate, ripe for a campaign of interplanetary conquest."
You see, Earth has one really big obstacle to overcome for simple life forms that didn't evolve here - free oxygen.
It simply doesn't exist normally in nature, the pesky gas is just too reactive, it bonds with damn near anything.
We breath it because some prehistoric organism started farting it out as a metabolic byproduct on our World, and for the rest, it was adapt or die.*
Seeing as how Oxygen is not of the free gasses detected on the comet, any lifeform that somehow migrates to Earth from there will simply not be able to survive our corrosive atmosphere.
*As a side note, adapting to metabolize oxygen really speeds up the old metabolism, and gives you the competitive edge on those sluggish methane and sulphur dioxide breathers**.
**Not to be taken as a slur against methane and/or sulphur dioxide breathers. Many of my best friends are anaerobic!