Now ive seen the pic I have to Agree, almost certainly Umbilical material, probably simply glued itself to his belly, unless, as stated its very young. (looks it/size dificult to ascertain) Definately DONT try to pull it off. If its quite dry, you could either mist, or better, dab softly with a wet cottonball a few times a day, untill you can determin weather its just dried stuck on cord, or is infact hernia.
A vet check couldnt hurt.
Re read edit: Way too young to do with mating, wrong place anyway

If it wasnt there when you got it, and its not dried bug guts, then you definately need to have a vet take a look. Something is wrong! Probably umbilical rupture, (thin skin over healed area has split after contact with something, and whats emerged has dried and the wound partially rehealed) is my guess.