Composting wood matter is broken down best by fungal activity. If you were to peel up that mulch, you would see hyphal growth that has consolidated into mycellium (white threadlike strands). It liberates nutrients from the wood mulch and feeds it to the surrounding plants, eventually producing short-lived fruiting bodies (puffballs, toad stools, etc.).
Nothing to worry about unless you've managed to keep things too damp, at which point, its the dampness, not the fungi that are the issue.
I currently have a mass of it in the half whiskey barrel I grow my peppers in. Due to the compost, earth and chicken mulch (composted chicken manure and wood mulch), I have a mass of them growing under the grass straw I mulch with. They'll be gone in about 5 days and have had no effect on the Anaheims and Passillas growing in the tub.