I'd be inclined to blame a leaking/blown capacitor at first, but I'm not sure there's one about (near that slot), and the slimy green bug angle could be, too.

The first stage of N800 disassembly (popping the faceplate off) should reveal evidence of any such infestation, and is pretty easy to do, and low-risk. Just do it over a table to catch the d-pad & buttons from the front panel; I always forget those are a separate piece until I turn it over and they fall off. Thoughtfix's video shows how.