A family of Microsoft word processing software products for creating web, email, and print documents.
WordMat is not compatible with Word 2016 for Mac. Here is their page where they mention Word 2007, 2010, 2013 and 2016 (if it doesn't say for Mac, it's for Windows), plus 2011 for Mac: System requirements.
You should be able to turn it off by choosing Tools>Templates and Add-ins in Word. Below Global Templates and Add-ins, select the add-in and click on the minus sign to remove it.