Two children appeared one day in the village of Woolpit, England in the 1100s. A boy and girl claiming to be siblings were totally normal, apart from having green skin. They spoke their own language and refused to eat anything other than green beans. Once they began eating other food they lost their green color, but the boy got sick and died after being baptized. The girl, once she learned English, explained that she and her brother came from St. Martin's Land, an underground place where everyone was green.
Sounds like a folk tale to me! But Wikipedia has some explanations if it turned out to be true: