You need to paint a no-grass texture at that place. Set the "Max Opacity" to 100% and the new texture should overwrite the existing one at once. Using an edit radius of 1 is a bit tricky as you need to paint directly over a vertex to actually change anything, so better set it to 2, and redo the outside-textures later.
Also use "I" to see which textures are currently used in that cell. There can be only 6 textures per quarter-cell (quad), a seventh one will be black until you remove another texture, if you try to paint an eight texture and this texture has grass or rocks, then only those tiny objects will be painted, and since they aren't linked to a displayed texture they won't move with the land. Deleting the 8th texture in the "I" menu removes the objects (you may need to move the land or reload the cell to see any effect.)