Baby and young chicks are the ones most affected by coccidia in the soil and droppings, especially when there are older birds around.
Please do not cull your flock because it might be Mareks. Get a necropsy done to ease your mind.
After seeing her age and her runny poop, I would have thought it was probably coccidiosis, which causes extreme weakness and dehydration. The drooped neck can be a sign of dehydration, Mareks, and other illnesses. I have doubts about Mareks, but Rebascora is a very good authority about Mareks...
I would separate her today to watch her, and put some paper towels under her to catch droppings. But keep her warm, so if you cannot separate her, just keep an eye out. Get some Poultry NutriDrench and give 1 ml daily. Make sure that she is eating and drinking. Look for diarrhea or bloody poo in...