The leaves are sweating. It’s a natural part of the respiration cycle.
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You will get that where your leaves overlap as well. I move my canopy about and target where the leaves touch as it could become a hot spot for mould 
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Keep exhaust fan on and a circulating fan moving air over the plants
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Thank you all 
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Like @Davyg says. I have had the same thing happen. Leaves touch each other / overlap and would be more noticeable if I moved a fan or forgot to turn it back on. Transpiration is what makes the plants engine.
If you seal a leaf in a plastic bag you can really see how much moisture it releases by the amount of condensate in the baggy.
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