i just took out my battery and i noticed there were some brown spots, like around the screws and some marks throughout. it looks like rust/buildup but i'm not so sure.
If the brown stuff is kinda crystalline, it could be that your batteries are leaking. You should replace the batteries. How old is the battery pack anyway?
It could be oxidation from the battery pack being older. I have never seen this from a Lithium Ion battery before, but ill assume any battery could have this happen to it. Touch it and see if it scratches off or if it has adhered to the screws. If the laptop is only 3 weeks old i would be marching in to my nearest Apple store and asking them to take a look at it, maybe it could be the internal battery is leaking or oxidizing, but im really not sure of the location of the internal battery on that model.