Green Smoke is a Custom Block effect available in the Workshop. It is a simple particle effect that makes a plume of greem smoke emits from the local Y-axis of a custom block.
The effect is also used with the teleporter made with Plating blocks, the difference being that if the player walks into the effect, the screen blurs and the player can teleport via the B and X buttons.
Green smoke was added in the 1.099b patch.