It's usually not a good sign when the egg develops mold. A lot of times it'll stop on it's own, but you might want to clean the mold off. I know some people who have had good luck by cleaning the mold with a dilute solution of anti fungal medication (a small amount of water with a few squirts of athletes foot fungus medication) Dip a paper towel in this solution and clean off the mold well. Then paint over the spot with band-aid brand liquid bandage. I'm not sure how well this works, I usually just let the eggs take their natural course. For me this often means that they go bad, although like I said sometimes they just get better on their own.