Ac issue

I have an air exchanger in a walkin crawl space. One of the ac vent feeds about 6 ft. from the exchange has mildew around the outside foil ventilation pipe insulation. What is causing this? How can I repair it?
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In another useless post from Home Moaners Hub, Jim writes:

What does the term "air exchanger" mean to you?

A bit of a contradiction there, don't you think?

Supply or Return?

What exchange?

Mildew is commonly caused by moisture.

Moisture.

Prevent the moisture.
Note that the water vapor capacity of a given unit of air is dependent upon the temperature of said air. If you reduce the temperature of said air, it can hold less water vapor resulting in condensation.
It is likely that the insulation on your ductwork is insufficient for the task (warmer humid air is condensing on the cooler surface of the insulation).
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Talk about eschewing obfuscation.
Your post is confusing double talk.
What do you mean by "air exchanger"?
Is it a heat exchanger or an evaporator coil or something else?
Sounds like a supply duct is too cold with insulation that is too thin leading to condensation.
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