Not positive but that last picture looks like eggs on her feathers. It could be lice and the crusty stuff is lice eggs. I'd put her in an warm Epsom salt bath and soak her to try get the most off her. If she's got them, your other ones probably do too so you'll need to treat all of them.
You could use permethrin dust for each chicken as that'll kill them, or it comes in liquid to make into a spray. You'll need to clean out your coop and treat that too especially the nest boxes. I've heard Ivermectin works for lice too, and we actually use that for scaly leg mites.
Repeat whatever you use in a week as they have a life cycle you have to completely kill or they'll be right back again. Keep checking and repeat again if necessary.
I'd give them vitamin water too as mites and lice are rough on their immune systems.