Ventilation ideas for under new addition

Peeked under the 20x10 addition we had added last month.
~2 feet of cement board was used to seal off the addition from the oustide. It has a stucco coat on top of it. There's one 2ft x 2ft access door that uses vinyl siding as a face. The siding is the perforated (small circles) kind to let air flow in. That door is by the steps, on the left side wall if you were looking at it from the back wall.
There is no other ventilation slots.
When I looked under there today, the blue styrofoam board that's nailed to the foundation beams was soaked in moisture and dripping. Te back wall uses 4 concrete forms with a flat 2x12 on top of them and 3 2x12's on their edge supporting the floor beams.
The flat 2x12 had a lot of mold on it. I took a bucket of hot water & bleach & crawled under & scrubbed it.
What could I use to add more ventialtion? I was thinking of trying to find 16" return vent duct covers, cut out a hole in the cement board and just screw them into the studs. Woudl that work? Any other suggestions? thanks
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Let me add that I'm not sure it's mold vs a fungus vs ....9whatever)
The addition is actually 20x20 - we took down the original old 20x10 addition & pushed it back 10 more feet. We left the original cinder block wall standing. The portion under the old 20x10 addition (i.e inside the cinder block wall) doesn't show any moisture even though it doesn't have any ventilation other than a 2x2 foot section was broken out so I could get from the new part to the old part.
saag wrote:

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