There is not such thing as a mixed breed glider. There are cinnamon gliders which are light brown, but they would still have a white belly. Almost any color variations will either show or not show (either be the color variation, or standard gray) so it's pretty safe to say she is just stained. Theres a chance her fur just hasn't had long enough to grow out and become it's natural pretty white. Or like sjones said, it could be pouch staining. Try switching out her sleeping arrangements more and see if that helps (not too frequently or she will over mark). Best of luck!