Mold in an orchid tank?

[MySerenity]

I just set up a 42 gallon fish tank a couple of weeks ago and my husband made a platform for me out of wood and aluminum screening. (It was the materials we had on hand at the time). We have a small computer fan set up in there for air movement. It's been staying around 80% and 80 degrees and the orchids seem to be very happy so far.

Problem is that I have mold developing around the wood (bamboo) slats. It seems like an obvious problem so I'm not super surprised, but I'm not sure what to change to fix it. If we had used plastic would that not be a problem?

I've also noticed a small number of little flies. Is that a problem? There's only been a couple.

This is all a new set up to me so I appreciate the help.