By default Forest creates a multi-sub with all materials of the Geometry List and assign it to itself, but you can override this behaviour turning off "Consolidate Material" and assigning your own material to the object. To recover the auto-material, turn on "Consolidate" and click "Reassign Material".
Note: there is a crashing bug in Forest 3.4.x turning off "Consolidate materials", when Points-cloud if the active display mode. Be sure to use other viewport mode before applying the above procedure. This problem is fixed in v.3.5.x, but it has not been released yet for Forest Lite.
Other approach would be to use a "Blend" material. Put the grass material in the slot 1, the matte in slot 2 and set the mixing value to 0. In this way you can switch easily between both materials with the "Swap" button. I have not tested this by myself, but it would work.