Sounds like a tapeworm. I had same problem about a month ago. I took my AST to the Vet and he gave it a shot of Dronzit to kill the tapeworm and its eggs. Seemed to work because I have not noticed any more white ribbons coming out of his vent. I would definately take the turtle to the vet though since under medicating will not kill the parasites and over medicating might kill your turtle.

I think anytime you feed any live fish (even minnows ) you risk exposing your turtles to parasites.

VS