Mochtroids are mutants genetically engineered by the Space Pirates in an attempt to clone Metroids. As a result, the Mochtroids contain the Metroid’s aggressiveness and basic energy-syphoning capabilities, but lack the tenacity and hardiness of their true cousins. These failed experiments were placed in Maridia, where they were best forgotten.
Mochtroids are incredibly weak. They will float over Samus, but can only drain a small bit of energy every second. They can be destroyed with any weapon, including the Grapple Beam.
The Japanese name for Mochtroid is “Metroid Modoki” (メトロイドモドキ), or “Metroid Mimic”. The English localization is an obvious pun on “Mock Metroid”. Could the Space Pirates have flushed these failed experiments down the drain, and that's how they got into Maridia?