The Fusion Material library is read-only, and you cannot add, remove, or edit anything within it. You also cannot edit material properties within any user-defined library. Instead, create a new material entry in your Favorites library, then add it to your user-defined library.
The Materials Browser dialog lists all the materials in the Fusion libraries and enables you to
View all the following properties of each material in the databases:
Add materials to your Favorites folder so you can find them easily
Edit material properties in your Favorites folder
Create new materials in your Favorites folder
Create new user-defined databases for your custom materials.