Hi

Welcome To BYC
It's hard to see, but are there nits at the base of the new feathers coming in? It could also be the sheath of the feather that I'm seeing. Look around her vent and under her wings to see if she has any visible mites.
It does look like she has been plucked a bit, but at her size and age she will go through some mini molts so she will be growing new feathers to replace what was lost.
If you feel like what you are seeing are nits at the base of the feathers, then treat her with a permethrin base poultry dust or spray. Since she is very young, a spray (liquid) may be more practical since you can use a qtip/cotton ball/eye dropper, etc., to apply if you wish.
Something else to think about - if your chicks are in a brooder, supply them with a dub of dirt, they will love taking a dust bath.
http://www.the-chicken-chick.com/2012/08/poultry-lice-and-mites-identification.html
https://www.backyardchickens.com/t/1151513/pesticides-approved-for-poultry