This is what shell disease can do to a hermit crab. Shell disease is a general term to encompass several diseases of crustaceans. Most forms are contagious and can be passed from raw seafood to your hermit crabs. This is why CSJ advocates you boil any seafood you plan to feed to your hermit crabs. Wild caught is less likely to be infected than farm raised but it’s not worth the risk of infecting your entire colony and potentially contaminating your substrate.
There is a chance this was an injury and not shell disease but the results are the same, a fused exo which prevents shedding during molt. In some crabs this results in death, in others missing limbs, or like SF a bum leg. The disease sheds with the exo but there is a possibility of cross contamination while the crab’s new exo is still soft. Read more about Super Fudge.