I solved the mystery
- Midori Boshi: Sargasso (緑星・サルガッソ, Midori Boshi: Sarugasso?, literally meaning "Green Star: Sargasso"): Usopp fires a Pop Green at his target, which then explodes into an array of resilient seaweed, strong enough to delay an avalanche of rock and earth. This was first used in order to protect the Thousand Sunny from an undersea avalanche.[10] The name is similar to the Sargasso Sea, a region of the North Atlantic Ocean where ships often become tangled in weeds.
10/10 chapter already.