EDIT: I tend to feel a comic is bad if you have to explain the punchline below it. But since people are confused about this one...
The green thing is an enemy from Minecraft called a creeper. All they do is explode and make your life miserable. Sorry if you haven't played Minecraft (what, are you living under a rock? Grab a pickaxe, mine your way out, then go play Minecraft!), the reference is kind of essential to getting the joke.
