The Green Army was one of two opposing factions that lived on a carbon atom in a miniaturised form. This was a result of a transformation devised by a Composition Adjustment Teleport. They were at war with the Blue Army for thousands of years (about eight hundred thousand, in fact). This period of time was lengthy as time was accelerated within their world. They primarily used Staser-guns.
When Adric was miniaturised, he was taken prisoner by the army. Daneman, one of the army's soldiers, and Adric were in turn taken as prisoners by the Blue Army. Both were rescued by the Fourth Doctor.