Slime Mold is not a disease, but it might look kind of scary.
It usually appears overnight after a rain storm, when the weather is hot and humid. The incredible thing about this is that this fungus grows pretty quickly. It has a bright yellow color, and it is completely harmless.
Sometimes, when mulch is applied to planting beds, this fungus is already present, but it is in its dormant stage, waiting for the proper conditions before it grows and spreads to other parts of the mulch. Slime Mold (Fuligo septicae), feeds on bacteria found in decaying plant material. In this case, it is found in aged mulch. The best way to control it is to let nature do its thing. You can use an iron rake and rake the mulch.
Do you have slime mold in your mulch?