I remember this from when my runners were little. I think it's just the loss of some baby down in preparation for feathering. Keep an eye on it. That is one reason to be sure they have the proper temperature away from drafts.
Thanks! That was my suspicion but I haven't had ducks before so I wasn't sure. They've been living in a big plastic tote in my laundry room but good grief do they get stinky and messy quickly. I moved them to a pool in my garage this evening. I'm hoping that will hold them ok for at least another five days until they go outside to our duck house.