I suggest you take the following measures as soon as possible:
1. Take the turtle out of the water and gently wipe the water off it with a clean cloth or paper towel.
2. Apply tincture of iodine to the place with long white spots for disinfection, and then apply drugs for treating skin diseases, such as erythromycin or chlortetracycline.
3. Put the little turtle in a dry and ventilated place to avoid humidity and high temperature.
4. Stop feeding indigestible food, and suggest feeding high-protein digestible food.
5. Observe the situation of the little turtle. If the condition persists or worsens, it is recommended to take him to the vet as soon as possible.
In addition, if the soft-shelled turtle does not eat for a long time, it may need to supplement nutrition by injecting glucose or intravenous drip. During the treatment, keep the turtle clean and avoid contact with other small animals to prevent infection.
Finally, I suggest you consult a professional veterinarian or aquarium for more accurate treatment advice.