Water fowl need alot more niacin than what is in Chick starter so to be safe and make sure they have strong legs I'd get them started on some Nutritional yeast or Brewers yeast [not baking yeast] 1 Tab per cup of feed till they are 10 weeks old. That is when they are growing the fastest.
If they are eating egg white than that could be the culprit. What my ducks and poultry eats always shows up in their poop and as long as it doesn't have blood in it I just think back to what they had been eating previously.
It's good to be concerned but if they are acting okay in every other way it is probably nothing to be concerned about You can also buy probiotics that are made for poultry look in your feed store or TSC