You might want to do a mild bleach soak, 10%. Once the mold spores leach into the stone they will regrow when the conditions are right.

10% bleach will kill most anything, soak for a couple hours, then soak in plain clean water, let air dry.

I have noted that setting the stones on edge dries them faster, I use a wooden plate rack, two wood slats with wooden dowels, they are inexpensive.