I am wondering if you have smoothing groups setup on your model. If you do not, then the polygon normals will not export correctly which can cause a grey sheen to appear over the model in Model Viewer (this may only happen under certain conditions but it's worth looking at).
I suggest you break the glass into it's only material, as Shawn suggested, and then make sure that smoothing groups are setup properly and re-export.
I suggest you break the glass into it's only material, as Shawn suggested, and then make sure that smoothing groups are setup properly and re-export.