What is this bright yellow stuff on the ground?

[Markcuda]

Run, don't walk, you are being invaded by aliens.:laughing::laughing:

 

[westbrooklawn]

Teg provided the correct answer... I see this a lot in damp hardwood bark mulch. the thing that amazes me is how quickly it can grow... will go from a quarter size spot to 12" round in a matter of hours.