Surely you ever noticed that some part of the potato sometimes is green. That part of potato is dangerous about your health and it is not desirable to eat. It is enough to cut that part and then you cooked the rest.
The green part of the potato is green because it contains chlorophyll, which is not dangerous, but it is a sign that that part of the potato contains large amounts of the toxin solanine, which serve to protect the potato against parasites and pests, but it is toxic for humans.
However, it should not to afraid you because you have to eat incredibly large amounts of green potato to fill sick and vomiting. A person who weighs only 50 pounds should eat even 8 pounds of this vegetable to feel something, but it is good to know that this part is not good for the body.