The green discolouration to a small part of the breast muscle is, unfortunately, a natural occurrence and sometimes found, especially in larger birds. This usually occurs in the area deep into the breast, shown in the picture.
Occasionally as the inner breast muscle develops during life it can do so in such a way that it interferes with its own blood supply.
The lack of blood to the part can then cause the characteristic green discolouration that you have found.
Although unsightly and quite perturbing the meat has no harmful effects and would not cause any problems if eaten.
Unfortunately the condition is not easy to detect until the breast meat is carved.