You can activate "pick" tool in Corona VFB toolbar and click on that plane in the VFB to select it in the scene (if you're running IR in the docked VFB, then there's no need to activate "pick" beforehand, you can simply click on any object in the VFB and it will be selected in the scene). I suspect it will be plane primitive with render multiplier scaling set to higher than default 1. Set it back to default, by right-clicking on the spinner, or delete the plane if it's not needed and the scene should render normal again.