Fungi of various kinds occur on and in organic mulch when conditions are favorable. Its presence is a normal part of decomposition and, although it may be unsightly, it usually does no damage. It's usually best to ignore it or remove and dispose of the fruiting bodies (mushrooms, etc.). Chemical controls to prevent fungal growth on mulch are not available. Stirring the mulch may promote drying and prevent fruiting.
Here are links to more information about fungal growth on mulch including steps that can be taken in the few cases when its presence is a significant problem:
Control of Nuisance and Detrimental Molds (Fungi) in Mulches and Composts
