It's important to remember that even vegetarian turtles will eat or nip at what they view as a free protein source, which would probably include things like Axolotls sitting on the bottom of the tank not doing a lot of moving. If it's a carnivorous species, it would probably actively pursue the Axolotl, some species will take on prey that would appear too large - particularly if you're thinking of one of the Aussie sidenecks.