Mouldy looking back!

Just wondered if any of you experts could help. I have four exbattery that I have had for a month now.  One developed a limp a couple of days ago, I can see a flesh injury which is minor but nothing else. She is happy to range and eating, drinking very well. This morning she is being bullied and I noticed her back, which is mostly naked is covered in what looks like a pale green mould.  It's a powdery, chalky pale green covering. I have tried to touch it but she is hard to get and I don't want her to damaged her leg further. The others are all pecking at it.  I noticed yesterday she had a dust bath but forgot to rinse as she was covered in mud afterwards, maybe it's the remnant of this? Just wondered if this was anything anyone else had experienced?

I agree she really needs to see a vet.