To me that looks like she's gots lots of new pin feathers coming out there, which is good, and with that many coming in, looks like she is finishing up a moult. However, are you sure you don't see any lice? They are skin/straw colored, often yellow, and oblong. They move fast but do hold tight to feather shafts. You have to lift up feathers and look fast. It almost looks like the yellow stuff might be egg casings. But, if you say you don't see creepy crawlies, and she's the only one with the issue, I can be wrong. Often hard to tell from a picture. So it could very well be a yeast infection of the skin.
If you do have lice or mites, you can get a poultry dust from the feed store. Seven dust or general purpose poultry dust will do the trick. Fluff them up with the powder, and repeat in a week for a few weeks and it should take care of it all IF they have that.
Are you sure they are 16-18 weeks old? If I am seeing a moult there... don't remember my point of lay birds doing that kind of molting at point of lay. Adult feathers coming in and replacing teen feathers is usually more subtile, although not always true. The way it looks... it almost looks like what my 2nd year hens look like when they go though a heavy moult and lose lots of feathers at the same time. Of course, I am only seeing a tiny portion of the bird, and it's just one bird.
Could you post a full body shot of your flock even if they look bad? I'm beginning to wonder if these may be older than you thought.
