Answer: What you describe sounds like a condition known as oedema (or edema). Oedema is a symptom of a physiological imbalance in plants. It comes from a buildup of water pressure in leaf tissue which can leak out (which is what you see on your plants) or can distort the leaves and burst the cells within the leaves.
Overwatering, high humidity, and low light intensities favor the development of oedema. Faulty watering practices, such as allowing plants to dry out completely and then drowning them, encourages it. Be sure to water regularly so the soil remains moist but not soggy wet, and try to reduce humidity or at least improve air circulation by providing lots of elbow room between your plants.
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