Fuligo septica

Scientific name:  Fuligo septica (L.) F. H. Wigg.
Derivation of name: septica means "rotten" or "putrefying" 
Synonyms: Mucor septicus L.  
Common name(s):  Scrambled-egg slime.
Phylum:  Myxomycota
Order:   Physarales
Family:   Physaraceae
Occurrence on wood substrate:  Occurring as slimy to
crust-like sheets or cushion-like iregular masses on stumps,
logs, living plants, and wood mulches in landscapes; May
through October. 
Dimensions: Masses are 2.5 to 20 cm long, almost as wide,
and 1-3 cm thick.   
Description: This slime mold first appears as a white to
yellow slimy mass with dimensions as given. The "flesh"
transforms into a crusty, cake-like mass of darker and
variable color. The brittle crust easily breaks away to reveal
a dull-black spore mass.       
Edibility: Inedible.  
Comments: Although many slime mold species fruit on
wood they do not form a penetrating and absorptive mass
of hyphae in the wood substrate. Rather, slime molds form
structures called plasmodia which are naked (i.e., without
cell walls) masses of protoplasm which can move and engulf
particles of food in an amoeboid manner. Slime mold
plasmodia creep about over the surfaces of materials,
engulfing bacteria, spores of fungi and plants, protozoa, and
particles of nonliving organic matter. At some point,
plasmodia convert into spore-bearing structures.
In Fuligo, the plasmodium converts into a cushion-shaped
mass of spores enclosed by an outer wall called a peridium.
This structure is called an aethalium (plural: aethalia). Fuligo
septica produces the largest spore-producing structure of
any known slime mold.

More information at MidwestNaturalist.com
More information at TomVolkFungi.net:

    
Figure 1. In this scene, the plasmodium of scrambled-egg
slime has crept all over this stump and even a polypore
fungus. Photo © William Roody.

Figure 2. Wood chips are a very common location for
plasmodia to start forming aethalia. Photo © Melissa
Emberger.

Figure 3. With its sudden appearance, bright yellow color
and conspicuous size, Fuligo septica is often quite
striking and grabs the attention of those who observe it.
Photo © Melissa Emberger.

Figure 4. This plasmodium on bark mulch is moving in an
upward direction in this view. Photo © Gary Emberger.

Figure 5. The plasmodium often spreads out in a fan-shaped
pattern over the supporting material.
Photo © Gary Emberger.

Figure 6. Here, a very large yellowish plasmodium
has converted to a very large aethalium, the
spore-bearing stage. Photo © Gary Emberger.

Figure 7. A closer view of the slime mold in Figure 6. Note
the darker and varying colors of the outer peridium.
Photo © Gary Emberger.

Figure 8. In the spore-bearing stage, slime molds are dry
and brittle. Breaking open the peridium exposes millions of
dry, dusty spores. Photo © Gary Emberger.