Hi mossioan,
Thanks for posting back.
According to your reply, I suggest you open the Activity Monitor (you can search for it with Spotlight). In the Activity Monitor, search for "SharePoint". You should see "Microsoft SharePoint". Click on it and then click the X in the top bar to stop the process. After that, please check if the pop-up will appear.

Also, if the pop-up window still appears, I suggest you try the following steps to look for this SharePoint keychain and remove it.
a. Close and quit all Office applications.
b. Go to the Finder> Applications> Utilities> Keychain Access. Check if you find this Microsoft Identity. If you can find it, please remove it. If you can't find it, please remove all Identities related to Microsoft.
c. Please restart your Mac and check again.
If the above methods don't help, please post back and let us know. We will keep on assisting you.
Sincerely,
George | Microsoft Community Moderator