I can't come up with a Green Valley Cemetery in Marathon County WI.There is one by that name in Shawano Co., but that's no where near Marathon Co.
For your information (and it may help), there is a Township of Green Valley in Marathon Co., it's on the border with Wood Co. and encompasses part of the Big Eau Pleine River reservoir.Almost half of the Township is wetlands and includes a portion of the Meade conservation area along the Little Eau Pleine River.
On the South edge of the Big Eau Pleine reservoir (we called a "flowage") at the intersections of county highways S and C is the remnants of a small community which we knew (some 50 years ago) as Moon.There might be a cemetery there, but it doesn't show up on my plat map of the Town of Green Valley.(Most of these places are the remnants of old communities, which generally had some sort of church located there.)
There is a Marathon Co. Historical Society in Wausau WI which may be able to help.But, for the life of me, I can't lay my hands on its address right now.
I'm sorry I can't be of more help, but perhaps I've given you a couple of directions in which to go.