A Skraaldian is a slug-like alien with a shell on it from the planet Skraal. Skraaldians come to Earth to multiply, as they like the warm climate in the sewers. In order to protect themselves, they wear large shells, which do not resemble their actual form. These shells protect them, as they are fairly defenseless without them.
If a Skraaldian is attacked or killed, it can vomit on the one responsible. That vomit then alerts other Skraaldians of what is happened, and they can lock onto the DNA of who is responsible. When in their shells, they are very powerful.
Skraaldian posess stingers that, while not fatal can cause those stung to 'swell up like an eggplant for a few weeks'.
They can nearly crush people with their claws. In order to get many around at once, Skraaldians also have a massive transport ship. the only thing worse than killing a Skraaldian is showing them your mucus. If you kill two they will put you in a large vat of soup at tempatures of 3000 to 4000 degrees.
In "The Long Goodbye Syndrome", Jay and Kay have to go in the sewers to stop Skraaldian multiplication. When Kay is attacked, Jay uses his Noisy Cricket to blow the shell off. The Slug comes out and vomits on Jay. When he returns to MiB, everyone throws him a goodbye party, as they know he is toast. After fighting off many slugs, the ship rises from the ground. Finally, Jay and Kay are able to defeat the Skraaldians, and Jay is saved for good.