Brown Algae (黒藻の獣, Kuromo no Kemono, lit. "brown algae creatures") are small plant-like animals which live in the sewers of the Musashi. They became friends with P-01s after her friendly gesture towards them.
Brown algae are animals with simple characteristics: they are often black in color with white eyes. They can communicate and understand human language, and appears to share a common consciousness with others of its kind. Another distinguishable trait of brown algae are their unpleasant scent, which is caused by their constant diet composed of sewage. They can purify themselves using a similar process as photosynthesis, but might dry up when left under the sun for too long.
Their consumption of sewage is deemed useful by the Musashi because they are able to purify the water in the process, and they are allowed to roam the sewage systems of the ship.
These critters first meet P-01s when she was cleaning the streets in front of the Blue Thunder Shop. Some of the brown algae, dried up from overexposure in the sun, asks P-01s to douse them with water. She agrees, and the grateful brown algae becomes attached to P-01s' kindness.
Within the days that follow, she is often seen talking to the brown algae while doing her daily chores.