It's not easy identifying a fungus in grass without close examination, but from what I've been able to glean from research-based information, your narrative and photo, my closest guess is some form of slime mold. The problem of exact identification stems from there being hundreds of species of slime molds. 

If you want an exact idea of what you have, I would contact your local county extension office, and they might be able to put you in contact with some fungi experts.

Slime molds don't do any damage, although large concentrations can cause the lawn to yellow. They generally disappear during hot, dry weather. There isn't any urgency to remove the mold, but if you want to clear it up before snow falls, you can wash or rake it from affected areas.

When the lawn is growing, frequent mowing can keep it in check.

https://extension.umn.edu/lawn-care/common-fungi