News for you and these are good ones
That aint shell rot, its either a calcium deposit or a erosion of the shell queratine plates(can also be both because the erosion of the shell creates a rough surface were calcium ions can congregate), both are quite comon and they "grow" with the turtle as you descrive. Its something completly benign and you have to do nothing about it, its a bit of a beauty prob only really, and this or next year when your turt sheds its shell plates it will disapear for ever. Shell rot its completly diferent
