Answer: Sounds as though your pothos has developed powdery mildew. You can remove the affected leaves, which will open the plant up to better air circulation, diminishing the chance of another bout with the disease. Powdery mildew is a fungal disease that is common, and will attack plants growing in stagnant, humid air. By increasing the air circulation around the plant, you'll keep your pothos less susceptible to the disease.
The gnats may or may not be related to the appearance of powdery mildew. Fungus gnats like moist places, so the soil surface of houseplants is a perfect breeding ground for them. Try covering the soil surface with decorative rock to keep the gnats from feeding and breeding there.
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