It isn't soft or smelly, It's just white. I also suspect it may be healed shell rot. I got her today, and the guy I bought her from said he checked it out at the vet. I'd still like to be sure.
It's just old scarring. Actually, the white you're seeing is dead bone. New keratin and bone are growing under that what patch and eventually the white will lift up and flake off. But it takes a very long time. It's not from shell rot. Its from injury.