They are cleaning the sides of their beaks, it also helps keep the beak in shape. the beak constantly growing so they have to 'sharpen it' to keep it from over growing.
Yes, perfectly normal. They do it when they are eating something "squishy" like yogurt or soft strawberries and it sticks to their beaks
I love it when they do that; I tell people who have never seen it that they are finished eating and wiping their beaks cleans from any food spillage.
