Foam at Turtle's Mouth
If this is an occasional thing after eating, or when he is biting on things, then it's just saliva, especially in box turtles. If it is persistent and all the time, your turtle might have a mouth/throat infection and should see a veterinarian for evaluation. It could be pneumonia, which is fatal if left untreated. To help him fight the infection, make sure your turtle is kept warm enough.