These are the smallest minifigures LEGO makes. Officially they are called nano figures or trophy minifigures. This one is the monochrome Sand Green colored one. It resembles a regular minifig except it fits on a single stud, and is not movable. It can be held by a minifigure claw hand however.
Collecting monochrome nanofigures
As of right now there are 12 different monochrome nanofigures. Here is a list of all the colors available.