When you add terrain details using only a 2D grass texture then Nature Renderer does not have any specific material to use for that texture. By default, Nature Renderer will create a new default material (internally) to use for the grass.
If you want to edit the material properties or use a different material then you can assign a custom material to the Material Override field in the Nature Renderer component. This material is then used for all the terrain details that use 2D grass textures. Click the "Create" button next to the field to create a new material in your project based on Nature Renderer's internal default material.
The main texture (Albedo) of the material is replaced with the grass texture, and all other properties of the material are kept the same.
2D grass textures are a convenient feature for quickly and easily adding terrain details. However, it is recommended to use a 3D model/prefab for more control over the detail's settings or materials.