Constant Yellow Square
 

Every time I start Excel on my MAC, a yellow square appears somewhere on the screen. It is not a comment or text box as far as I can see since it appears before I have opened any spread sheet. It just appears and can be moved by dragging and dropping but I cannot delete it.

When I close my spreadsheet it stays on the desktop until I click somewhere else on the desktop and then it goes.

Any ideas what it is and how to get rid of it please.