What should I do about the white, rotting shell under my turtle’s neck, even though the water is fine and it’s eating normally?

Shell rot in turtles is usually linked to poor water quality. In this case, keep the turtle dry (out of water) for periods to lower the risk of bacterial infection, and let it bask in sunlight, which has a disinfecting effect and aids healing of the plastron. For the affected area, rinse with saline solution, disinfect with povidone-iodine, then apply a chloramphenicol (aureomycin) ointment. Severe shell rot also needs antibiotic treatment. Afterwards, keep the water clean to prevent reinfection.