its a fungal thingamajig from what I understand and it "lives" off the planets natural resources or something, and it ejects toxins that kill all other life on the planet...
the big one is a "spore chiminey" or something, and if its disturbed (shot etc..) its supposed to release spores that screw up the atmosphere...
basically, its a really nasty very advanced fungus....
its living, but not really moving... think of like a bloody mushroom with spikes and an exoskeleton....