I think there's something going on there. See how the perimeter of the shell has those teeth? Someone who knows what they're talking about better will chime in soon.
It's not shell rot but it looks like the scutes are not done shedding. The marginal scutes (the outer ones) are sometimes the last ones to go. By the looks of it, it does look like the turtle grew a little too fast as the shell should be smoother and the jagged marginals. Those should come off eventually and after a few sheds, the shell should look very good. The green is algae, and you can use a damp cloth to wipe it off or use a new toothbrush and go for a gentle scrub.