They really need a good feed [not bird seed] especially when so young. Are you in the US? much easier to find feed etc here than many other countries. At around 3 weeks is when you may start to see problems with their legs, having problems with weakness, bowed legs beginning and then lameness because at this age niacin deficiency is showing up. Waterfowl need niacin to keep this from happening and rolled oats and birdseed isn't going to give them what they need. Also ducklings that are growing at such a fast rate of speed need protein also. Duck feed has basically all they need some can still show a niacin deficiency even on the best feed but it must be more of not being able to assimilate the niacin in their feed for the particular duckling. So many of us will also add a niacin supplement to their feed, Nutritional yeast is one, liquid B complex is another and the best if one is already showing problems with their legs.
That poop could be from something they ate in a veggie you gave them.