Was it moving? If it is an internal worm problem, it should still be alive and might move. You might have to poke at it to see if it will move. Also could be a lot of things. I had to pull a piece of grass or weed, I didn't inspect too closely out of my little Leo's butt once. It wasn't as wide looking as in your pic, however, that could just be the close up shot. I wouldn't worry unless your tort shows any problems. If your worried though, get his stool checked.