Why does Wilma have green on her butt

[uglyduck]

]I have a 2 year old Brahma (Wilma) that has a spot of green skin at the base of her tail feathers about 2 inches
above her vent. My first thought is gangerine. She acts and looks very healthy otherwise.. i visually checked for lice and mites and found none. also none of my other 8 hens have this condition....... Could someone give me an idea of what it could be and how I can treat it.

Thanks

 

[charsbird]

Green skin on chickens is bruising. Could she have been injured?