That patch of grey is a blank info window. Enable mouse cursor with F10 (if it's not already on), alt-click on it to select it (alt is a modifier key, so that you don't accidentaly click on windows when trying to click on cockpit switches...clicking is for switches and alt-clicking for info windows) and once it's in focus/selected right click on it. You'll get a pop-up menu with options, you can see an option to close it.
For more information on customizing your overlay interface windows, check my frequently asked questions thread for a solution, it's the first topic mentioned:
http://forum.1cpublishing.eu/showthread.php?t=24782