What made you think they would fall off? Any areas of concern? I would leave it be until a problem arises.
There are some areas of concern w/ your tank. The blue calcium sand is bad for the crabs and can cement/stick to their bodies, which can hurt them if they try to molt in it. Also, Kaydee is right, I would definitely get many more shells of different sizes, this could cause aggression.
There doesn't seem to be anything else glaringly obvious. Really, I wouldn't worry until they give you a reason to. Good luck!
