Termite indicator
The person who posted this image had no idea what they were looking at. It looked like minuscule tennis balls were inside of an electrical socket, but for what purpose, nobody knew. Of course, he uploaded the image to Reddit to seek the truth. And he was set free after discovering what this mysterious object really was.

It’s, for lack of a better term, a termite indicator. Below the green balls are wooden fibers that termites gnaw through because they think everything wooden is delicious. If the termites gnaw too much of the fibers, the balls will fall and disappear behind your drywall. Some might consider this a too-late-time-to-move indicator.