That grey stripe is the edge of the default Skybox image used in older BIMx app versions. We have already changed this background in all the mobile applications, please install the latest build. The next update of BIMx Web Viewer will also include the new Skybox.
Also, you can easily set a different background for a 3D model in Archicad. More info can be found in the Export Options for a 3D View described in this article.