First off, find out what the material is on the slab that is being displayed. Select the slab, go to "Edit Type" in the Properties. Under Structure row, pick the Edit button. Find the top material. Lets say concrete for our example. Pick on it and select the "..." button. This will open up the Materials. The select material should be pre-highlighted. Select it and go to the Surface Pattern area on the right. Change the material to
. Select OK three times and you will be back to your view, which is now updated to not show the surface material.
Note: This will remove the surface pattern for all of the slabs with that same material. There are some other options to duplicate the slab type and material, if you want slab "A" to show surface material, and slab "B" not to show the surface material.