What is this odd Excel file doing here?
This is a known bug in Office for Mac 2011.  The icons represent temporary files that have been deleted.  (Probably before they were deleted they were invisible.) You cannot Get Info on the files or find the files via Terminal because they do not exists; the bug is that the Finder is still showing them.  If you force-quit the Finder it will restart and the file icons will be gone. 

The best workaround to keep them from coming back seems to be to move files off the desktop and into a folder before opening them.