Try Romaine lettuce and chicory. Also get algae disks (the kind that are for pleco catfish and other algae eating fish). Most pignosed turtles will jump on those. Fruits are good. Try some shrimp as well.
You may have a fungal situation here. Raising the temp to 80-82 degrees can help remedy the situation. Also use one tablespoon per 5 gallons of AQUARIUM salt NOT TABLE SALT AND NOT MARINE SALT. Use the type of salt that is used for fresh water tanks.
A trip to the vet is the best thing to do though. Perhaps it may just be a blemish on the shell rather than actual fungus but you need the advice of a qualified medical person to make that determination. Good luck.