Was she vaccinated for Marek's?
How many chicks have you lost, what is the timeline on that?
Can she walk, or does she lie down most of the time, is she unsteady when she walks?
Since she isn't laying eggs, I would feed the chick starter, limit the scratch/grains/treats to no more than 10% of her daily intake. (provide grit for her)
The green poop is a concern so providing good nutrition may help. Offer some vitamins/electrolytes in her water. Nutri-Drench or Sav-A-Chick are usually available at the feed store.
For feeding the rest of your flock, provide them a grit source free choice, just put some in the run and let them pick up what they need.
Since you have different ages in your flock you may want to consider using an all flock/flock raiser feed, it's easier than having to buy multiple products and the birds not in lay don't need the extra calcium. Provide oyster shell free choice, those that are laying eggs will take what they need, the others will leave it be.