it is just some slime mold that is feeding off the decaying mulch…not a problem for the plants. Rough up the area with a rake and let the mulch dry out (stop spray watering if your are) and it will dry up and disappear when the heat and humidity subside. Here’s a link to more: http://www.gardeningknowhow.com/ornamental/fungus-lichen/what-is-slime-mold-slime.htm