I can't offer much advice from medical side as I've never dealt with neurological problems in ducks...
But in crested ducks, neurological problems, such as seizures, is unfortunately common. Crested ducks have a hole in their skull, which is where the tuft of fur comes from. It is possible the duck is having seizures from a treatable medical condition, though, so stay tuned for a duck person with more understanding of how to treat these.
First - I would separate her in case she is sick, injured, or could be getting picked on (pecked at her eyes, chased away from the food), especially since she's in with chickens who are notoriously peck-happy. Make sure she has a bowl deep enough to cover her eyes in to clean them. If she isn't eating as much make sure she gets electrolytes or nutrients (nutridrench) in her water. I would coax her to eat by mixing treats (lettuce, cucumbers, peas) into her food.
How is her poop? Color, solidity? Duck poop should resemble what they eat - if she has eaten a lot of grain it should be a more solid, brown color. If she is foraging grass and drinking a lot of water it could be liquid and green. However, if she is not eating grass/plants and the poop is green and runny it could be diarrhea.