"Milk of manganese" is not a recognized chemical term. It is possible you are referring to manganese dioxide (MnO2), which is sometimes called "black manganese oxide" and is a compound.

Here's why:

* Element: A pure substance made up of only one type of atom (like gold, oxygen, or carbon).

* Compound: A substance formed when two or more different elements are chemically combined in a fixed ratio (like water, which is H2O).

* Mixture: A combination of two or more substances that are not chemically bonded. They can be separated by physical means.

Manganese dioxide (MnO2) is made up of two elements, manganese (Mn) and oxygen (O), chemically combined in a fixed ratio. Therefore, it's a compound.

If you're referring to something else, please provide more information!