What is the black liquid? Is it safe?

Maitake mushrooms have a natural black pigment to them, thus making the liquid they produce black as well. It is not scum; it contains nutritious ingredients beneficial to health, like polyphenol.  To get the most nutrition out of the mushroom, do not discard the liquid. One helpful suggestion is to use Maitake mushrooms in soups for creating a highly nutritious broth.