To get this achievement, you must find this tiny alien and click on it. He's very minute and can be hard to see. If you take more than a few seconds, he will run away, squealing like a pig. I also believe he might spawn in different places.
This achievement can be unlocked at the very start of the game, as he spawns inside the middle cabinet in the large living room where you talk to Rose on the phone. (The right most cabinet of the two right by each other.) He will spawn on the shelf with the white, decorative vases. You can open the cabinet doors without him reacting.
If he doesn't spawn here, exit to the main menu, start a new game, and look again. The chances for him spawning here are relatively high, so you shouldn't have to do this more than two or three times before you see him.
Just click on him, and he'll start to glow. The achievement will pop!