Micronoids are microscopic alien creatures that originate from another dimension and are the masterminds behind the alien invasion of Mega-Primus. They exist in the bloodstreams of all aliens, and they utilize psionic powers to control the minds of the host.
The Micronoids knew that the races they utilize cannot survive on Earth, so they seek to assimilate the humans in order to escape their dimension. Micronoids are transmitted to human hosts via Brainsuckers, which latch onto their chosen host and inject Micronoid Aggregate into the bloodstream, allowing them to take control of the host's mind.
By themselves, Micronoid Aggregate is no more than a mass of slime made up of billions of Micronoids. Their combined psionic powers allow them to conduct psi attacks, but they are very weak, and it will take a lot of time for them to mind control an X-Com agent. But all in all, as a unit, they are easy to kill by any means sans psionics.