Here is a closer up photo.

That is Fuligo septica or "Dog vomit slime mold"

No, I didn't make that up. It's the common name for it. It doesn't hurt anything. I just ignore it.

Here is a link that might be useful: Fuglio septica

It's just a fungus that is breaking down the mulch. Not harmful and there is nothing you have to do about it. The fungus that grows in or on mulch is usually either benign or beneficial to plants.

If you want to read about the many different kinds you can google 'fungus on mulch'.

Here is a link that might be useful: Here are some photos that look like yours ...