It’s a myth in American culture that every family has a coat of arms or a “family crest” or at least one exists for every last name. However, coats of arms are granted to individuals, not surnames.
In other words, if you don’t have documentation that says thay you (and you alone) are authorized to display a “family crest,” you are not authorized to show it.
Details can be found in a YouTube video produced by Genealogy Magazine, entitled “Episode 10: The Family Crest,” available at: https://www.youtube.com/watch?v=du4SLAImGGs.