Thanks for the question.

It is a fungus. It is feeding upon your mulch which I assume is rather moist. It will not affect your plants. Don’t eat it. But what it is exactly I’m not sure. I’m going to guess that it appeared rather suddenly. My speculation at this point based upon your third picture is that it is an emerging puffball. You will know if this is correct or not within a few days at which point it will get very large. If it does so, please get back to us and we’ll provide you further information about it.

Thanks for asking us.