Please use the stock icon GTK_STOCK_EXECUTE for the "Execute" menu entry. http://git.xfce.org/xfce/xfdesktop/tree/src/xfdesktop-file-icon-manager.c#n1465 xfdesktop 4.11.5 (git/master)
Change pushed to master in: http://git.xfce.org/xfce/xfdesktop/commit/?id=850fd27b6bf4384a4ae5c5a94d66e8f775900ef0