OK, I see 1 picture, a bit blurry. Does the white come off if you gently rub it and if so, is the white cottony? Is the area soft?
I'm also looking at the back right of the shell, the scute to the right of the very last vertebral scute (which is the last scute going over the spine). Is the outer edge of that scute turning white as well, or that just in the picture?
Without knowing the answers to the above, if more than 1 area is turning/has white on it, I'd suspect a fungus or *possibly* the beginning of shell rot. If the white is only that one mark, it's possible the turtle somehow scraped or otherwise injured himself. But I'd wonder if some bacteria had gotten under the scute and if there wasn't the start of something like shell rot there as well. If you go to
http://www.austinsturtlepage.com/Care/medshell.htm there is some info and pics on different shell conditions. Does anything there sound/look like what's on your turtle's shell?
"You become responsible, forever, for what you have tamed." -Antoine de Saint Exupery-