They cannot be treated (a fungicide will not be effective) and treatment/eradication is also not necessary. This kind of fungus is merely decaying organic matter in the soil (or the mulch itself) and it is not causing damage to live plants. Mushrooms are a bit like flowers or fruit on plants -- it's how they reproduce -- and their removal doesn't take away the entire organism. (If you pick flowers or fruits off a plant, the rest of the plant remains and stays alive.) Once the mushrooms fade away (or are plucked off), the rest of the fungus "body" remains in the ground as fine threads that consume organic material. This activity recycles nutrients back into the soil that live roots can use. Some fungi that produce mushrooms in lawns or garden beds are classified as mycorrhizae, which are a group of beneficial fungi that help roots absorb what they need in a symbiotic relationship.

If you are concerned that a child or pet will try to eat the mushrooms, you can cut them off or scrape them away with a shovel and throw them out, but that won't get rid of the fungus itself. Once the weather dries out more, the mushroom production will slow or stop.

Miri