The right thing is Grey's. Lacks it the apostrophe or apostrophe ('). It is an English term meaning "Grey", according to Grey on Grey's. Does Grey's Anatomy reference, i.e. the television series Grey's Anatomy (Merdith gray and his fellow doctors).
As I do not think that they are asking about color in English, or dolores of some names, I guess that it is a mistake by the plural of grey, which is 'flocks'.
It is an English word which means grey, rucios. It is also gray, bleached hair.