Im not 100% sure how shell rot really looks either, but it usually happens on the bottom of the shell. His/her shell just looks a little dusty. Have you tried scrubbing it with a toothbrush during a soak?
Someone more experienced will come along and state whether or not it is.
Mina, shell rot happens when they're kept too wet. This very unlikely to be the case with any of your torts. This is definitely not something you need to worry about.