That is a slime mold. It is a fungus, and typically manifests itself during wet weather. There is no good cure I know of, just remove the mulch from the area and see if you can reduce the moisture, less irrigation, etc.
Preventive chemical treatments tried over the years have been found ineffective. Slime molds get their nutrients from dead organic matter, such as mulch. Although slime molds may grow on plants, they do not harm plants. Slime molds will eventually disappear on their own. If you want to speed this process, rake the mulch to promote air drying.
Also, use a high pressure stream of water, hose, etc to remove the slime mold and it will help for a bit.