Fuzzy green spot on yolk

[tjcschwar]

I have 4 female khaki campbells who were born in May and are laying well (about 3 eggs a day). This morning I cracked open an egg that had been laid during the night, and it had a light green spot on the yolk that looked like mold. It was about 3/8" diameter and looked fuzzy (although I'm sure that it would not have been if I had touched it, coming from inside an egg). This is the second egg that we have gotten that way - the other about 2 weeks ago.

Does anyone know what that spot is?