You may want to get some Medistatin (nystatin) and treat them for a fungal infection. I would do the recommended warm soaks to the bottoms either with Betadine and a little dish soap, or Epsom salts water once a day. Dry well, and apply an antifungal cream daily. Those could include Nustock cream from a feed store, clotrimazole from Dollar Tree, or Monistat 7 from a drug store.
It also looks like there has been some feather picking about the tail, and that can lead to cannibalism and vent injury or gleet. Hopefully, the feathers will grow back after the next molt. I would make sure they are getting enough protein in a balanced layer or all flock feed, and are not overcrowded.