This is slime mold. After all the warm, rainy, humid weather, this can occur. It is harmless to you, children and pets. It feeds on organic matter but doesn’t damage turf. Fungicides aren't necessary for slime mold.

Temporarily decrease irrigation and rake the area to promote air drying. When the turf is completely dry, mow the affected area to remove slime mold.

Here are some links with more information on slime mold:

http://planttalk.colostate.edu/topics/insects-diseases/1435-slime-mold/

https://www.extension.purdue.edu/extmedia/bp/bp-112-w.pdf

http://www.plantpath.k-state.edu/extension/documents/turf/slime_mold_june_2015.pdf