That’s an interesting behavior. Is this a fresh install or did this just start happening recently? If recent you can always try a system restore to a day it was working properly. Install all Microsoft service packs and updates. If that doesn’t fix it then disable all add-ins.If that fixes it then re-enable one add-in at a time till you find the rogue add-in. If that doesn’t help run a repair of Office. If that doesn’t work run an uninstall/re-install of Office. Call MS for support if that fails as well.
Right click on the icon and change the display settings to not turn to 16 bit or uncheck 256