Many new setups will cycle through this
Hyphal Growth is the very first cycle of 'mold' development.
For example, formation of spores begins when ie a piece of wood is exposed to different levels of light, oxygen levels, temperature, and nutrients.
Some woods are less prone to this than others.
Prepping the wood can help/ interrupt a cycle that may have previously started (place the wood in the oven 225 degrees F 30 min )
Mold spores can remain dormant for years..... even in hot and dry environments.

Let it cycle
As long as it drains properly and has some ventilation is should disappear.

So... yes ... you need a drainage layer

:butterfly