The Mollusk is a very slimy giant green slug without any eyes. Her size varies, as she is ever growing, a trait that causes her to constantly seek a new shell. Her first shell was indigo with bits of a pillar stuck in it and her second shell was constructed of school supplies.
She is first seen being hunted at night by Steven and the Crystal Gems. While Steven is walking, the ground suddenly moves underneath him and it is revealed to be the Mollusk, whose shell was made out of the pillar pieces Steven had stepped on. The Crystal Gems quickly poof her and Steven grabs a piece of her shell to bring to class for Show and Tell.
When it is time for Show and Tell, Steven realizes that the Mollusk's shell is missing along with various school supplies. Following a green trail of slime and supplies, the Mollusk is soon found in the gymnasium. Connie leads her classmates in a fight against it with dodge-balls being thrown at her weak spot, the shell.
Eventually when the Mollusk breaks through the walls into the cafeteria, high sodium lunch is thrown at her causing her to shrink due to her slug characteristics. Although she is now small and slippery, the Crystal Gems are able to arrive in time to poof her, but at the expense of the school which subsequently collapses from battle damage.
Constant Growth: The Mollusk is constantly growing in size, something that requires her to seek a new shell often. When bigger, she is more difficult to defeat and, while smaller, she is harder to hit.
Projectile Launching: The Mollusk is able to launch projectiles from the materials acquired to make up her shell, such as pencils and chairs.
Shell Formation: The Mollusk is able to create a shell from any material as a means of armor to protect herself. This armor also doubles as a weak spot, however.
Slime Mimicry: Having a body consisting of slime, the Mollusk is able to slip away from attacks easily as well as cause her enemies to slip in her slime, such as Mrs. Lezner.
The Mollusk's biggest weakness is salt, as it was used to shrink her and make her easier to poof.