Dark spot top of crayfish's leg

[Pterrys Mom]

Pterry molted about 4 weeks ago, and since then, I started noticing a small dark spot on the undercarriage in front of his back left leg. It has been getting progressively bigger. I think that leg seems to be not doing as much as it used to. The local pet store did not recommend a salt bath like you would do for shell rot. I'm attaching a picture. Has anyone seen this? Solution?View attachment 1243088 View attachment 1243088 View attachment 1243088 View attachment 1243087 View attachment 1243087 View attachment 1243088 View attachment 1243087