Mua

Mua (the Acid Worm) is a miniboss Samus fights in Metroid: Zero Mission. Once Samus has activated the rail system in Kraid's Lair and attempts to move onwards, she will encounter this creature. Mua resides in a considerably large pool of acid.

Battle Guide
Mua attacks by lunging out of the acid at Samus, spitting globs of some sort of substance (presumably more acid), and raising the acid level (not present in Easy mode). The worm will flick its teeth in and out just before it lunges at Samus. To avoid this attack, Samus 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, Samus should jump to one of the two higher platforms in the chamber; however, if Samus has performed a Sequence Break to obtain the Varia Suit early, the acid is nothing to worry about.

To cause injury, Samus must fire at what appear to be the creature's eyes just after it has stuck itself in a platform after lunging. She needs to fire from directly opposite the creature to be able to cause injury. The most effective weapon to use is a Missile, but Samus can still use the Ice Beam if she has run out of Missiles, which is most likely to happen on Hard Mode. If Samus happens to have the Wave Beam or does not have the Long Beam, she won't be able to cause any damage, either because her shots are too large or because the shots won't reach. To replenish Missiles, she must crouch near one of the hatches on either side of the room, and wait for the worm to lunge at her. Since it can't reach her at the door, it will spit globs of fluid at her - shooting these globs may produce Missile ammunition.

As Mua takes more damage, it will strike more quickly, forcing Samus to act faster. Once it has been defeated, the acid will drain, allowing her access to the floor. A path leading to a Missile Expansion lies under the spot where the worm was resting.

Trivia

 * The Mua cannot be battled unless Samus arrives after activating the rails throughout Kraid's Lair. If she does, the room will be flooded with acid and several Reos will be present.
 * The battle with the Mua is somewhat similar to that of Serris, as both battles involve some sort of locomotion on the ceiling to avoid the enemy's attacks.