This green color is not harmful to the health of the eater. If you boil the egg too much, the taste of the egg will not be as good as when it is boiled just right, and the egg will be drier, so it will not be as soft and creamy as when it is boiled just right or boiled with the yolk.
If you do not want the green layer on the yolk of a boiled egg, simply do not boil the egg for too long. If you like the rich taste of hard-boiled eggs, stop at 11 minutes and turn off the heat.
If you want your eggs to be well-done, boil them for only 9 minutes. For soft-boiled eggs, 3 minutes is a good amount of time. You can wait 5 minutes if you want the yolk to start to thicken or 7 minutes for the yolk to be cooked but still pink in the middle.
