Anthony
Mold eh? Welcome to the Pacific coast.
The most readily available mold fighter is household bleach. It will kill the mold and bleach the wood to remove some or all of the stain depending on how dark it is. Use a fairly strong solution (1/4 cup to gal of water) or stronger as needed. You will notice that the bleach gives the wood a yellowish color, which you may not care for.
Another solution is oxalic acid. This available thru chemical supply houses, paint suppliers and some hardware stores it is also the active ingredient in many commercial deck/wood bleachers. Oxalic will give the wood a more orange cast than bleach which I think looks more natural.
Over time the natural darkening of the wood will obscure the color changes caused by either method. It is also best to treat the entire piece and not just the molded and stained areas. And I have learned that sanding the affected areas before bleaching seems to improve the effectiveness.
Final note: none of this is magic and you may have some residual stain left. You can try sanding or planing but my experience is that bad stains go deep and repeated bleachings will help but at some point its best to leave well enough alone.
And don’t forget eye protection
good luck
