Muriatic acid is used to clean concrete (sold at hardware stores) but be careful with it (strong powerful stuff) and be sure not to mix it with any other chemical, which reminds me why not try a solution of household bleach and water first and see how that works, you may have to do a little scrubbing, then rinse it with water.
A bit offtopic, but muriatic acid is also used to maintain the pool water.
Oh, sweet lifeguard memories.
Vinegar and Pine Sol both work on mold, you can dump those on and let it soak down into the concrete and see if that helps, too. It's a lot cheaper and less harmful than muriatic acid, so maybe that would be a good place to start first.