Marasmus is a vessel rented from Pallas Station.
The Marasmus was discovered by Naomi tethered to Eros while the Rocinante and the Guy Molinari set up the demolition of the asteroid. Their first presumption is that those aboard were pirates there hoping to plunder items for resale from the ill-fated asteroid.
Alex sends a drone to investigate the Marasmus closer. Naomi overrides the airlock remotely to allow that drone access to the interior decks of the ship. The Rocinante crew members are surprised about the state of the med bay of the Marasmus, spacious and well-equipped. Amos comments that pirate ships frequently just utilize an open airlock as a med bay. Before the drone acquires more evidence, it is abruptly disabled.
Holden orders Amos to prepare to board the mysterious interloping Marasmus but Doctor Mesplede from that ship initiates a connection request before Holden gets halfway down to the deck below. He is the activist doctor representing Aid Without Allegiance as seen in a broadcast during "Safe", the first episode of the second season when Rocinante first left Eros. Mesplede admits that he overreacted when he disabled Rocinante's drone, and is under the impression that the Rocinante is a Martian ship.
When Holden first inquires about the presence of the Marasmus, Doctor Mesplede conceals the fact that his crew gained access. Mesplede shares a story about being on a humanitarian mission of mercy to help the survivors but emphasizes the asteroid is locked down. Holden attempts to intimidate Mesplede with the threat of boarding his ship but, just lets the crew stir a while before releasing them.
When Miller discovers one of the airlocks was hacked and breached then finds the body of a Marasmus crew member infected by the protomolecule, the Rocinante crew realize they have been deceived and order the Marasmus return and subject themselves to inspection. Rocinante jams broadcasts potentially originating from Marasmus. The Marasmus refuses to submit insisting that their obligation is to reveal the information to the whole system about the protomolecule bioweapon they found. Holden then confesses that the Rocinante is not with the MCRN and the protomolecule was not of Earth or Martian origin and begged Mesplede to keep his silence, reasoning that people would come to investigate and unleash an outbreak. Mesplede declares if Holden truly is on the same side, he knew what he needed to do. Marasmus attempts to go around Eros and evade the Rocinante's jamming range.
Holden launches the torpedo destroying it before Marasmus can get away.
Marasmus, by definition, is a child who suffers from low birth weight, for their age due to severe undernourishment.