3 year old Red Cross with green pasty bottom, odd stance

I lost one of my 3 yr old Red Cross a week ago that had no visible signs of distress (other than it was clearly the smallest chicken that did get picked on and seemed a little thin). Today one of my girls has a green pasty bottom, which I tried to wash with warm soapy water, and seems to stand funny with legs spread apart. Her comb appears to be not as bright and is smaller than the remaining 4 healthy birds. They are usually kept in an enclosed run but I let them roam in a larger fenced in area on the weekends when I am working in the garden nearby so possibly she ate something??? Any thoughts or suggestions? I have six new 5 week old chicks which are in their own adjacent, enclosed area in the coop. Just moved them in the coop 5 days ago and they seem to be fine and healthy but certainly don't want anything spreading.