Mites can cause anemia, then death. Visually inspect your hens for mites, especially around the vent area.
Since you've wormed them with fenbendazole, and now their feces is green and runny...there's a possibility of an ecoli infection. The only way to find out for sure is to have a feces sample submitted to a vet or a laboratory for analysis.
Check for mites first.
I totally agree with a fecal sample. It doesn't usually cost that much and can really help you get a handle on things before your new arrivals