You can chain to this effect, so you can target your own face down card and then activate it. (this works with cards like "Mystical Space Typhoon" and "Call of the Haunted" or "Book of Moon") this being said you cannot activate most cards in the damage step which is when "Greenkappa's" effect is likely to resolve.
If this card is attacked, then your opponent cannot activate "Starlight Road" in response to this card's effect, as you cannot activate "Starlight Road" in the Damage Step.
If you activate "Widespread Dud" after this card flips up, you can destroy that, and also get rid of 3 cards your opponent controls.
You can also use this card in an "Artifact" deck to destroy your set "Artifacts".
If you do this, wait for your opponent to attack this card and don't flip summon it or else your destroyed "Artifacts" won't get their effects.