It's very hard to see in the picture. It could very well be an attached yolk sac, in which case it will fall off soon and leave a little stump. In rare cases, the umbilical area doesn't close properly when the gecko is developing and there is an abdominal opening through which the intestines protrude. It's happened to me twice in 13 years of breeding. In this case, the gecko will not survive. I'm betting on the yolk sac. Let us know what happens.
Aliza