How to make stair with grasses
I believe its turf and not real grass. You have also asked how to prevent this from moving or falling off. So in case it is real grass ; grasses Belo to the category "runners" (it is one category among the sub aerial modifications of stem). Runners don't have the ability to cross obstacles. So the edge of the stair is its boundary and it will not fall off. However if it grows long it will require mowing or cutting with a scissor.