Crayfish hatch from eggs held on the underside of the momma's belly, and are held onto by the swimmerets. There are typically hundreds of them, and from hatchling they look like little tiny carbon copies of the adults.
What you're seeing is probably a parasite as you suspected. Crayfish and other crustaceans are a very important intermediate in the life cycle of MANY parasites. Most notably, and like what you're describing, are Cestodes (tapeworms). There are too many different types to list. Luckily for you, the tapeworms can only be passed to the turtle if the turtle consumes the crayfish. Suprisingly, even if the turtle eats the worm itself it should be fine.(Same phenomenon with dogs and cats; they can only get tapeworms from eating fleas).
If your turtle has been eating live crayfish, it wouldn't hurt to have it dewormed. Crayfish should be offered frozen and thawed, which kills the parasites within. A turtle who eats live crayfish should be dewormed by a professional on a regular basis.
Hope this helps. Take care.