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Ah. . . I see what you're worried about. We see this frequently, and because the tortoise has to open his mouth to eat it's a difficult spot to get to heal. Without having seen it happen, we're left to guess. Something caused this tortoise to open his mouth too wide and it tore the corner of his mouth. Maybe he caught his beak on something he was climbing and he slid down and wasn't able to close it in time, we'll never know.
Pick off all that hard crust and clean it up as best you can then dab on some first aid cream. You want to keep it soft and not allow a hard scab to form. Neosporin, or even a little Vaseline would work. You'll have to be attentive and do it daily.
When you feed him, give him chopped up food, things he doesn't have to open his mouth too wide for.