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As the Cray gets more comfortable they will color up better and also when in a darker substrate and back ground. They will color up a little to match the surroundings.
The speckles look normal but can't see well in the pics if that is what you are referring to.
There are also fungal disease called dark or black spot and they could die from it.
Also they can get a bad spot and molt off the dark spot, I would remove the molted shell part from the tank if he has that.
I was referring towards this brown spot on the tip of the claw. The dark substrate was put in about a month ago so adaption makes sense. Just don't want my little buddy to die!
BTW your link to the picture of the claw takes me to a thread about a clown fish biting the owner haha.