Sounds like stress colouration. My gibby does that a lot because hes a bit of a wuss (he only does it when we go near the tank, or try and take his picture), but it can also be an indication of other stress causes such as water condition, incorrect setup (eg. bright lights and nowhere to hide), etc.
What size tank do you have? I know it's probably not relevant while he's still small, but he's going to grow quick, and grow huge, and the recommended setup for an adult is 5ft x 2ft x 2ft minimum. Not that that's relevant to this topic, so I'll shurrup now *lol*.
