Slime mold, out of all that are attracted to wood mulch, is fairly common, not harmful to humans, and it looks a lot like dog vomit. Yep, that's even the nickname for it, and it normally appears in either white, yellow, orange or black. 

As was shared on the "black slime mold" page, appearing overnight was about a square foot patch of black slime mold on the playground EWF, finding and shoveling it out from the playground early every morning wasn't the solution, rubber mulch was, but we had to find out why it was happening and what preparation had to take place. The lack of proper installation and maintenance rendered the playground safety surfacing unsalvageable. 

Slime mold can have a bad and lasting effect on sensitive plant and tree roots. As the founder of this company shares on the "attacking plants" page, one community learned this lesson the hard way, when it was discovered that the wood mulch they were spreading around to beautify and protect was actually killing their trees, shrubs and flowering plants... to the tune of $2 million to replace over two years.