macOS's Launch Services Database is responsible for linking files with the programs that created them.
I do not use Microsoft Office products so I am unable to determine if it can fix that particular problem or not. However, if you are willing to attempt fixing the problem you can rebuild the Launch Services Database with the Terminal command that follows below. Since I am not experiencing that problem the only assurance I can provide is that the procedure works as described on a properly functioning Mac, that is, one without any mysteriously incorrect icons.
You are probably already aware that Microsoft doesn't support Office 2011 any more so if you were to contact them they won't want to know you. Their solution will be to upgrade to whatever bloatware is their current version. That might not fix the problem anyway. Nor is there any guarantee this procedure will fix it either.
Proceed at your own risk and back up your Mac prior to doing any of this. To learn how to use Time Machine please read How to use Time Machine to back up or restore your Mac - Apple Support. Should something unexpected occur, your only recourse will be to completely restore the affected system from that backup, so you must be absolutely certain you implemented a viable recovery strategy. If you accept that responsibility, continue below.
Open the Terminal app. It is in your Mac's Utilities folder.
/System/Library/Frameworks/CoreServices.framework/Versions/A/Frameworks/LaunchServices.framework/Versions/A/Support/lsregister -kill -r -domain u -domain s -domain l -v
Select the line above (triple-click to select the entire line), then Copy and Paste it into that Terminal window. Do not attempt to type it. Then, press the Return or Enter key to execute it.
lsregister will take a few moments to finish. It will return many lines of text including various "errors". That is only an indication of progress and it is not worth paying any attention to them.
After it finishes your Mac will run slowly for a few moments, with a corresponding increase in heat and energy consumption. Every Finder window that you open will be slow to populate as each one rebuilds the icons they contain, which also takes time and is CPU-intensive. That only has to occur once though.
When lsregister finishes the problem should be resolved.
Or, you will render your Mac unusable and you will need to restore from the backup you just created, so be sure to have a reliable backup strategy as I initially suggested.
The reason the problem occurred is another question altogether. Needless to say, if you are using any non-Apple "anti-virus" or "cleaning" products, anything at all in those broad categories of garbage that should never be installed on any Mac, don't. Many things won't work if you do.
