It may be nothing, or it may be a sign that the duck is feeling tired, or has some leg problems that are just beginning.
I would make sure to give vitamins/electrolytes/probiotics, and be sure that the duck is getting enough niacin.
Since you have chickens and ducks together, are you feeding them all chick starter? Have you moved on to grower yet?
Ducklings need three times the niacin that chicks do. Not all ducklings appear to have trouble with low niacin, but a number of them develop leg and nervous system problems.
So if they are all on chick starter, I would put 100 to 150 mg of niacin per gallon of water for the next several weeks. Don't use the no-flush, fancy kind, just plain niacin capsules emptied into the water. An alternative is a sprinkle of brewer's yeast on their food in the morning.
I would also check the feet and make sure there are no sores or cuts or splinters.
Time in lukewarm water deep enough to float in can help stretch and strengthen muscles and take weight off of sore legs or feet, too.