I am an owner of four crabs and the one I have had the longest now we named Champ has been acting weird. I don't know if he is sick or pre-molt thought so I'am very worried.He hasn't expierenced any of the storeing up on food or blackish bubble things yet, but he has been very lathargic (exuse my spelling) he won't even come out for me to see his eyes or antenne so I'am not sure exactly what is going on.I'am really worried thought, I haven't seen him come out in a while but I always check to see if he is still alive every day or so. I just wish he would come out so I can see him and make sure he is okay, he is just not himself.
If your crab isn't acting like normal and all the other ones are (which would rule out environmental conditions) ISO that crab quick! It could be ill, or it could be pre-molt, but either way, it needs time for itself. Give it everything it needs in its own space: fresh and salt water, a hidey and food, and leave it be. Just check in once in a while, and keep the food & water changed.