This effect is a conversion of my Construct 2 effect of the same name.
You can test a live example on Scirra Arcade.
I have such an error: i.gyazo.com/ee56e22d0c9684bb004682579e033520.png
It means what it says basically. You already have a different effect that have same ID as mine. My effect it's quite old so it's using simple ID: "fog". If you want it to work you need to change it to something else... or contact the author of other effect to change it.
Thank you for answer, I changed id