We live by the beach in south Florida and had a similar experience.
We have the bigger 160 and the great Cookshack cover for it.
We are in a nightly saltwater storm and high humidity the rest of the time.
The CS is tightly sealed,especially with the cover.
I had been doing most of the cooking on my FEC,to practice for comps.
I love to cook on the 160,so I opened it to do some butts, and it had mounds of cotton like mold.
I cleaned and hosed it well,and sun dried it.
Cooked the butts the next day and it was like normal.
Since then,I put an Al 1/2 pan from Sam's filled with cheap cat litter[activated charcoal ?] on the middle rack and haven't had a problem since.