Why go small?
This moon has been infected... ...in fact - it is infected for millenia. These creatures need a lot of time to evolve - they are pretty harmless (except the fact they make their nests from moons and planets that have molten core).
Their eggs orbit around the nest (moon/planet) for really long time until they hatch. They probably don't need air or any other known elements to survive. All we know is that they prefer planets/moons closer to stars and need a lot of heat/energy to give birth to the young ones.
//Inspired by Farscape series. Living Leviathan ships gave me this idea.
Edit: Added some details here and there, changed colors here and there. Nothing dramatic.