This cannot normally happen if you uninstalled our products in the proper way (from the Windows Control Panel), since our uninstaller surely removes the FSDT Exclude area from the Scenery Library.
However, if you uninstalled in any other way, or some of your configuration files were corrupted by something else, so our Uninstaller couldn't remove that area, it could cause that but, of course, the easiest solution would be what the sim suggest: unload the area, so the message will go away.