It's a reference to the game Minecraft. In Minecraft you collect and make blocks like stone, wood, dirt, etc and use them to make houses, castles, or whatever else you want. There's a day/night cycle, and at night various monsters, like zombies and skeletons, come out and you have to survive. The most infamous monster of the game is the creeper, which is the green thing. It makes absolutely no noise until it gets right up next to you, at which point it makes a hissing noise (kinda like a lit fuse) and blows up a few seconds later, generally killing you and destroying a lot of blocks in the surrounding area.
They're notorious for sneaking up behind people and scaring the shit out of them shortly before they blow up and destroy whatever the person was working on.