Also do not feed her any grass or starch, like bread. They cause sour crop. You can feed it to chickens on normal, but not to chicks with sour crop. Best wishes to you and your chicken!
Well, you have done right by separating her. Then you just need to stop feeding her anything and add apple cider vinegar to her water. A little bit of garlic will help also. Watch this:
P.S. I think the apple cider vinegar also helps with mucus and mucus might mean it is a respiratory disease.