The outer layer of the yolk turns green when the egg has been slightly over boiled. I have been told that has to do with a chemical reaction involving the sulfur in the yolk.
Don't worry. It's been overcooked. If you boil a bad egg, you won't be able to tell the difference till you taste it. It's also better to boil eggs closer to the expiration date because the membrane inside the egg has started to separate from the shell and makes it easier to peel.