When you say that she isn't walking, can you show a picture of how she is sitting? Do you mean she is just sitting more than walking around, or does she ever walk around at all? If she can't use her legs at all, that is a very serious new symptom.
Slowly becoming unable to walk over time is a classic sign of Mareks disease due to leg paralysis which comes on gradually over a period of days or weeks. Watch her movements closely for changes in balance and dexterity. If it continues to worsen or if she starts sitting with her legs splayed out, then it is most likely Mareks.
Mareks disease also causes tumors, though usually on the inside. Read through this article and see if any of these symptoms match what's going on with her:
https://www.backyardchickens.com/articles/the-great-big-giant-mareks-disease-faq.66077/
It may not be Mareks, but familiarizing yourself with signs of this highly contagious virus will help your whole flock.
The only way to know for sure whether or not she has Mareks is to send her to a lab for a necropsy. You may want to call the nearest veterinary lab and see what the process would be to test her in case she does pass away. Knowing what she has will help you care for the rest of your chickens.
If it is a wound, you could try getting some vetricyn spray and spraying the area with it daily to encourage healing and prevent infection. If it doesn't seem to be improving after a week or so of epsom salt soaks and spray, then you know it is not a wound or an infection.
If it was just muck, it would be coming off in the water with the epsom baths, so it's definitely not just mud or poop.
I hope she pulls through. But in case she does pass from this, it is best to be prepared mentally for what the next steps are to protect the rest of your flock from getting this if it is contagious.
