First, get those pots off the ground and into stands.  Even fs the stand is only 1" off the ground it will allow air to flow underneath and this will prevent the problem from reoccuring. 

First you need to use a scraper to scrape off as much of the mold as you can.  Don't dig into the wood so that it gouges, just scrape away the surface of the mold.  Aways go with the grain of the wood.

 

Then use a sanding machine and 180 grit sandpaper to sand the area.  If mold remains, clean the area with a little bleach dilute in water.  Be careful not to leave the solution on or use too strong of a solution as this can change the color of the wood.  Next sand with 200 grit sand paper and apply a layer of finish.