I will like to confirm Lizardman's comments with a personal experience similar to yours.
B4 cure, we need to know how it got there first. In my own experience, such whitish 'rot' on the carapace particularly with soft-shelled turtles, is a result of poor water quality. As water quality deteriorates for several reasons : poor or inadequate filtering system, overcrowded, etc. acidity of water increases. You will need to invest in a cheap ph tester and do a weekly check on your water's ph level to ensure that it does not drop below 7.5 with 7 as neutral. I keep mine at 8.0 to 8.5.
I have 3 pig-nose which had just recovered from the above symptoms. Treatment is slow and may take up to 1 month.
Immediate action : total change of water (always use age water and anti-chlorine) wash filtering system and all rocks, tank ornaments thoroughly, and then i added 'Aquadine's' general aid for fin rot, body fungus, etc. (easily available at your pet store) and treat my turtles with this medication for 2 weeks before letting the remaining 2 weeks for own healing. Important is the maintenance of water ph.
I fed normally and within 3 weeks, all my piggies are back to normal.
u can write to me should u require more info at russchie@yahoo.com