Mua (also known as the Acid Worm) is a large worm that resides in Kraid's Lair. Mua resides in a considerably large pool of acid.
Mua attacks by lunging out of the acid, spitting globs of some sort of substance (presumably more acid) which contain Energy Capsules or Missile Ammo when destroyed, and raising the acid level. The worm will flick its teeth in and out just before it lunges. To avoid this attack, one must use the rail system on the ceiling to travel to the other side of the room. When this happens, Mua will ram into a block hard enough to accidentally lodge its teeth in the block, making it unable to move for a few seconds. When it raises the acid level, one should jump to one of the two higher platforms in the chamber.
To cause injury, one must assault what appear to be the creature's eyes just after it has stuck itself in a platform after lunging. One needs to fire from directly opposite the creature to be able to cause injury. The most effective weapon to use is a Missile. As Mua takes more damage, it will strike more quickly, forcing one to act faster. Once it has been defeated, the acid will drain, allowing one access to the floor. A path leading to a Missile Expansion lies under the spot where the worm was resting.
- Metroid: Zero Mission (First appearance)