A family of Microsoft word processing software products for creating web, email, and print documents.
Sorry for Falola's wasting your time with a Windows answer to a Mac problem.
Mendeley has been obsolete in macOS for a long time, so I don't have an example installation to give you the exact folder to check.
Close all Office programs. In macOS, hold down the Option key while clicking on the Go menu and choosing Library. Your hidden user library folder opens. Open ~/Library/Group Containers/UBF8T346G9.Office/User Content. Check both the Add-ins folder and the Startup/Word folder. For any file that has Mendeley in the name, drag it to your desktop, then restart Word to check if the problem is gone.