I presume the plywood is to hide under?
As for teh mold if you just took cypress fresh from a bag I;m not surprized. Gardening cypress is usually damp in the bag. its stored outside, rain etc it will be damp. Add that into a warm "tub" I'll presume again a rack? There is not enough ventilation for the damp cypress. An aquarium with a wire lid would probably be ok. I've used bagged cyress alot. But never in a rack.
I would just dry the cypress out throughly. Like back it in the oven at 250ish on a cookie sheet. From teh people I spoke to at excelisor sp? woods aspen is kiln dried as it is used for industrial appz, and the bedding is kind of a by product, although their sales people will tell you it is not. So any ways, "proper" kiln dried lumber is 4-8% moisture. That cypress could feel kind of dry and be 20% .
Dry it out and you may solve all your problems. I have a lot of black cement tubs and i would set mine out in the sun in the summer. OH, I had a tramopline too,that is a great place to dry stuff on a sunny day.
Good luck
merry christmas
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Thanks for reading.
Big Tom
www.HerpZillA.com