Either the roof leaked and caused the carpet to gain water or the slab foundation is seeping moisture. You are going to have to peel back the carpet and cover it with a piece of plastic. Seal the edges thouroughly and wait a few days. If condensation appears then the moisture is coming through the slab. If it remains dry, then the moisture came from above. In either case the carpet will have to be replaced and a thermal barrier installed before the new carpet and padding are installed.I know of no way of removing the mold from carpet safely. Maybe others....