I am using Microsoft Word for Mac on my MacBook Pro OS X (Yosemite). I tried closing a Word document but clicking on the red button made it turn gray and nothing happened. I can minimize the Word document, but can't move it to trash nor save the document.
In Mac MSWord with the document open and the three buttons x + - not showing you seem to have no facility to close the document without quitting Mac MSWord.
You can go to the File drop down menu and select Close file. The active document closes.
Mac MSWord remains open, so you can continue work on other documents. It is my experience that not all Mac MSWord documents I am working in present without the no x + - buttons phenomenon . So there seems some underlying issue ...
When the buttons are visible the File dropdown menu does not yield this Close file option.