Those appear to be maggots (it's a little hard to tell, the picture is kind of blurry) which means she has flystrike. Flystrike is not pleasant. I lost a drake to flystrike because I didn't catch it early enough. Here's the link to that thread detailing what I did:
https://www.backyardchickens.com/t/...-found-out-its-flystrike-and-update-on-my-boy
Anyway, the insecticide I used was permethrin. Here is it here, but my bottle was smaller:
http://www.tractorsupply.com/en/store/permethrin-10-livestock-amp;-premise-spray-32-oz
It's safe to use right on the bird, which is what I did. I diluted it A LOT and put it in a pan of water, then put the duck in and sloshed it over him. Within minutes all the maggots were dying and falling off into the water. Just don't let her drink the water. I lost my boy because I caught it too late, but the important thing is to get the maggots out right away.
Before you just go and treat her though, look her over and make sure she actually has maggots, since the picture was blurry and I'm not 100%. It will be disgusting, and they actually make a noise when they're all squirming, so be prepared for that. Look her over really well, since they could be under her feathers. Especially check near her vent, because if some poop got stuck in her feathers that could be what caused it in the first place.
