Hi Paulina. This thread was not really about feather problems, but about a feather mutation that occurred in some Ameraucana chickens. You would get better help posting your picture with a description in the Emergencies/illness/disease section of the forum.
You need to identify what type of mites or lice the chicken has to find the best treatment. There are several good articles on BYC or you can do a search on the internet for poultry lice and mites. No matter what type they are, you will need to treat your entire flock. Personally, I would start with some poultry dust to kill adult parasites. You will have to retreat when those eggs hatch, probably in a few days.