Seaweed Assault is an Atari VCS game by Random Terrain.
Your name is Captain Nalad Modnar and you are part of an advanced underwater civilization. It’s your duty to patrol your section of the ocean in a semi-organic submarine called the Manatee. Highly aggressive seaweed surrounds you and it keeps trying to replicate faster than you can shoot. Use your torpedoes to blast any bits of seaweed that appear before they clog up your sector and spread to others, making it nearly impossible for anyone to travel. Don’t let the seaweed strangle your civilization to extinction.