not a clue but dont discard it just incase there is life in there but maybe seperate it from the others incase it is some sort of bacteria or fungus type thing
I agree with Sexy chef- keep seperate if possible but try to hatch anyway.
Only suggestion I can think of is that the calcium is insufficent in that one so the egg hasn't formed the outer layer properly. That's just a guess though so don't just take my word for it.
A bit late reaction :P 10 of the eggs have hatched 1th of july 4 females, and 6 males Maybe 1 or 2 holdbacks, 1 female who got a large spotted area and completely black belly. :D pics comming soon