The white stuff in your grass may be fungus and not mold. Ducks have a good sense of taste and they will not eat it. Most the fungus in the grass is not toxic anyway. Another thing that comes to my mind is grit. Are you given them any indoors? They need that to digest their food and to absorb nutrients. I have a 5 pound bag of builders sand from home depot and they always have a small 4 ounce jar of that indoors. I start that one day 2 in the brooder. Now they don't get any sand, since they are outdoors everyday. My guess is that your ducks are now getting the grit they need outdoor and therefore they are improving fast. Just a thought. Vitamin water with electrolytes are also important. Mine got it the first week daily and not they are getting it once per week.
Katharina
Katharina