There are many things that can cause that color of poop; not eating enough, bacterial infection, moldy feed, a liver problem, respiratory issue...just to name a few.
Start with the basics first. Check your feed and make sure it isnt moldy, ensure there isnt algae growing in waterers. Check and see if there are mold or fungus in bedding or any other place that she could have access to, breathing in spores or eating contaminated feed.
You can take care of not eating enough; purchase a large syringe without a needle, poultry nutri drench at a feed store and mix 5 drops of the nutri drench in a mixture with buttermilk and scrambled eggs...all into a slurry that you can syringe feed your hen 4 times a day til she can eat on her own. The nutri drench is loaded with vitamins and minerals, scrambled egg for extra protein and buttermilk as a probiotic. It is better than yogurt. Yogurt is good but tends to run out of them, buttermilk coats their intestinal lining and absorbs easier.
If it's a bacterial infection, you'll need to purchase baytril, most likely from a vet and dose per the vets instructions. Dosing is done by the weoght of the chicken. If it's a respiratory issue, the baytril will work on that as well.
There's not much you can do for liver problems. I recommend you check the feed for mold, then start her on the slurry mixture that I mentioned. Hopefully she'll pull out of it. Give it 5 days for some improvement, if not, get the baytril and dose her accordingly and continue the slurry.
You can always take a fecal sample to a vet and have it analyzed. Good luck.
