It's not behind a planet, it's on top of it like usual :) The green is Corrosia's Slime Trail, fully upgraded, which you can manually aim to create what I call a "slime fortress" like this. Then just turtle inside it, enemies can't get through, they die trying. At least up to Wave 120 Victory.

Behind the planet?

how did u maneuver your ship behind the planet like that? no matter what i try, i can't figure it out