This morning I snapped these photos on the way out the door. I spent the weekend working in the garden in this exact spot so I know this was not there yesterday. It appeared overnight. This is a classic yellow slime mold and they are one of the most fascinating species on earth.
The term mold implies that it is a fungus but technically this isn't true. While slime molds have a lot in common with true molds and fungi they are classified separate from them. There is still work to be done to correctly classify them all and scientists don't even all agree on the correct classification.
I'm pretty sure this is a Fuligo septica, one of the plasmodial slime molds. It is in the class Myxomycetes. If you're a person who does better with common names than scientific names perhaps you'll prefer the colorful common name, "dog vomit slime mold". Fuligo septica commonly grows on mulch or wood chips. I didn't think this area was very wet but my wife may have watered after I went inside which triggered the growth.
What you actually see in the photos is called an aethalium, it is similar to the fruiting body stage of a mushroom. This growth will eventually darken and release spores which is how slime molds reproduce. Before this stage, the slime mold is a plasmodium. These are truly strange and where slime molds get their name. A plasmodium is comprised of thousands of individual nuclei that are attached together in one large membrane filled with cytoplasm. Collectively they act like an amoeba and actually move in search of food. This is why scientists have had such a hard time classifying them. They act like a fungus and have spores but they can move and act like a simple animal as well. When French botanist Jean Marchant first described the species in 1727 he classified it as a sponge. He clearly didn't see it in the plasmodial stage! Check out this incredible time lapse video of a plasmodial slime mold in action.
If you look around you may find a slime mold near you. They are found pretty much everywhere on earth.