It looks like muscular necrosis to me, and fairly advanced… I’m not sure much can be done for him at this stage, to be honest :c You can try adding some indian almond leaves or alder cones (if you don’t mind the tannis), both are beneficial for shrimp health (and also the shrimp like to eat them). Also it would be good to check your water quality and oxygenation in the tank, as usually this pops up if the shrimp are stressed or weakened by some water quality issue or other stressors in their environment.
It would be good to separate him in particular from the others if at all possible though, the main thing is you don’t want them to eat him when he passes, as that can potentially spread it to them.
Separate_Ad_3027 
It looks like muscular necrosis to me, and fairly advanced… I’m not sure much can be done for him at this stage, to be honest :c You can try adding some indian almond leaves or alder cones (if you don’t mind the tannis), both are beneficial for shrimp health (and also the shrimp like to eat them). Also it would be good to check your water quality and oxygenation in the tank, as usually this pops up if the shrimp are stressed or weakened by some water quality issue or other stressors in their environment.
It would be good to separate him in particular from the others if at all possible though, the main thing is you don’t want them to eat him when he passes, as that can potentially spread it to them.