Hi Breezy. I don't see where you mention you have a drainage layer such as stone or hydroballs.
Is the mold on the wood or in the substrate?
Mold in the substrate will or should cycle and go away. On the wood, I think you would need to remove, wash, and dry out complete in the oven. Bake at 200 degrees for about 1 to 2 hours. Check it routinely so it doesn't burn up.
If you don't have a drainage layer, your substrate will saturate and rot quickly. To crate a drainage layer, you must use about 1 to 2 inch of stone or hydroballs, cover with a mesh screen and then cover the screen with the substrate. The moisture will drop slowly down through the substrate and into the drainage layer to evaporate.