Small spot on musk turtle

Does anyone know what this is? I noticed a small abrasion on my 11 year old male musk turtle, I felt around his shell for any signs of shell rot but it’s still hard, I’m really scared it might be something else. He’s still active, swims, eats, and basks occasionally, I just don’t know what it could be. I’ll still probably get it checked out, even if it isn’t serious.