Green poop, need advice

[Emmapee]

I have a buff orphington who has been pooping this bright green/white poop lately. She had been acting normal until a few days ago when she started to become really lethargic and was puffing out all of her feathers. I brought her inside to isolate her and keep the others from getting sick if it is contagious. She has been up and down. She's been talking to us more when we check on her and she has been walking around. She really has not been eating, or drinking, which is the biggest concern. We know that she is an older hen, but we don't know the exact age. Everything I've read online so far has said green poop equals death, pretty much. I was hoping someone had some advice... What could it be and is there any hope? How worried should I be about it spreading to my other girl? I'm new to this BYC thing and I'm totally lost. I would be devastated if it spread to my other girl and killed her... ANY advice or information would be really appreciated.