Does it look like your dog threw up on your mulch? Are you confused by that because you don't even have a dog? While it may be the pet's fault, it's likely that you have a fungus or mold in your mulch (this specific one is the aptly named dog vomit slime mold). Other kinds of mulch fungus include birds nest fungi (they look like tiny little cups with seeds in them) and mushrooms.
Mulch fungus is unattractive, but it doesn't hurt your plants. Mulch is just dead wood chips, so it makes sense that fungi would show up to decompose it. In fact, we want to see mulch decomposing over time to help improve the soil. We just don't like it when that decomposition stares us in the face in the form of ugly molds and mushrooms. Different species of fungi attack dead and living plants, and the fungi in your mulch won't attack your living landscape. You may sometimes see the slime molds on your plants, especially if a patch on the mulch has spread onto a plant, but the mold isn't in your plants to harm them.
Controlling molds and fungi is pretty easy: you just use a spade or trowel and remove the top layer in that section of the mulch. If the mulch underneath is a lighter color or has white threads running through it, those are part of the same fungus colony. You can dig them out as well to slow down the reappearance of the above-ground fungal structures, but if it's a huge area you may prefer to just break up the colony and rake the mulch smooth again. The mulch you have dug up should be removed from your garden or put in a compost pile to delay the reappearance of the above-ground fungus. If you have a persistent mold and fungus problem, you can consider mixing in some mushroom compost to help control it. Now this won't get rid of all fungi in that area forever. That would be impossible, and not desirable since we do want the mulch to slowly decompose. Our goal here is simply to manage the unattractive appearance of some of these helpful decomposers. If you want to learn more about specific kinds of fungi in mulch, check out this bulletin from Penn State.