On MBP running Sierra my icons on excel (2011) files has changed to a program icon I used in the 1990's . I believe it was called Zen of Mac. Looks like a little "Jack in the box". See screen shot.
See if this works: select an Excel file in the Finder, Get Info (command-I), go down to the "Open with" popup and select the Microsoft Excel application, and then click the "Change All..." button.
See if this works: select an Excel file in the Finder, Get Info (command-I), go down to the "Open with" popup and select the Microsoft Excel application, and then click the "Change All..." button.