Slight mold in the substrate is harmless. Only if the entire surface gets covered in a forest of mold, then you might worry.
The "food" in the substrate, sugars and the like, will break down no matter what you do if you keep any moisture in the tank at all. It is far better to increase the air flow to keep it off the surface than to just watch it happen all over again once you change it.
Dribble your water down the side of the tank/container/enclosure and the water will keep the bottom layers moist and the top can then be dryer. This keeps the mold off the surface, and yet allows the animal to burrow to a depth where it is comfortable.
IF you want to not see mold, you'll need to compost your media, a suggestion Michael Jacobi made, very wisely, in his DVD on introductory tarantula care.
First, pour yourself a glass/cup/stein of your favorite beverage and clear your schedule for the next little while.
Then, click the Search link on the gray bar across the top of this page. In the Search drop-down menu, type or copy-and-paste mold wood substrate into the "Search Forums" box. Click <Go>. I got 167 hits.
Or, in the Search drop-down menu, click "Advanced Search." In the resulting window, use mold in the "Keywords:" box, and Pikaia in the "Username:" box. That would produce about 13 postings that I, personally, have made over the years on this subject.
Feel free to use other search terms as well. For instance, using simply mold will get you enough hits to keep you up most of the night. Fungus will get about the same number but mostly different results.
Somewhere in there you should find what you're looking for.