I would use search box top right for lots of threads on crayfish and turtles.
Note some freshwater crayfish has a short life expectancy , no more than 2 years.
When molted will become more of a target for your turtle.
Regular crayfish get 3-4" but electric blues can get 6-8".
Known for carrying disease.
They also are great climbers , excape....
Grown somewhat attached ? If so may want to offer it a home of it's own to live out a full life / if a regular crayfish a 10 gallon tank is good. Even with your turtle never eating live food all it takes is once. Also turtle was there first , his home , may get territorial and hurt the crayfish ?
http://www.redearslider.com/prey.html