The significance of this interaction is open to interpretation. While it was very surprising to see a typically very composed Michelle flustered, it does not to appear to be due to Melania breaking with tradition. You can watch footage of both below and decide for yourself.
P.S. I think the true faux pas was Donald leaving his wife at the car as he raced up the steps. Arriving together is the well establish polite protocol. Perhaps he should ask Barack for some relationship advice!