Each year we know that with the summer comes a vast array of slime molds (also known as dog-vomit fungi) appearing in mulch. They appear in several sizes and colors with no definite shape.
I have watched those in my mulched landscape beds change on a daily basis from a bright yellow netting, to a tan powder, to a dark brown dried blob.
Slime molds get their nutrients from dead organic matter, such as mulch. Although slime molds may occasionally grow on plants such as turf, 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.