Wood landscaping mulch is made of shredded wood and bark. Given time, microscopic bacteria and fungi cause the wood to decay. Slime mold, or Fuligo septica, sometimes referred to as "dog vomit mold," is one common fungus that can grow on decaying wood. The mold is unsightly but causes no harm to plants or animals. The best thing to do is to leave it alone and allow it to finish its cycle of decay. You can, however, take steps that may prevent the formation of mold on mulch.
Toss or rake the mulch. Do this every few days during periods of high humidity coupled with high moisture, such as after rainfall or watering. This may help prevent the formation of mold or stop it in its early stages.
Carefully remove mold if you prefer not to leave it alone. Beneath the mold can be spores that will spread. Because areas of mold are usually small, you can cover and scoop the mold with a plastic bag, such as a garbage bag. Push the sides of the bag beneath the mold, collecting the mulch along with the mold.
Dump the mold and mulch in your compost bin or close the bag and place it in the trash.