The term "sea slug" is actually a general term to describe anything that looks slug-like with no observable shell and lives in the sea. This can include any number of invertebrate groups world-wide, including but not limited to: Nubibranchs, Sacoglossa, or Sea Cucumbers (Class Holothuroidea). Other than "slug-like" and "shell-less", sea slugs come on a vast variety of forms and colors, and diets vary tremendously as well.
In the Franchise, sea slugs are portrayed as short, black, eel-like creatures. They are even shown to have eyes, a mouth with lips, and two tiny fins behind their heads. This runs counter to any creature in real life termed a "slug", and somewhat implies an internal skeletal structure.
In actuality, some creatures that fall under the blanket term "sea slug" are also edible to humans. Sea cucumbers are highly prized in Asian cuisine. They are also used in traditional medicine to treat anything from cancer to arthritic pain, but so far no scientific studies back these claims.
Feeding habits. If we can figure out what this thing eats, then we might figure out where it eats. ... We've come across a few islands out there that are just crawling with sea slugs. Hopefully, our Razorwhip's feeling hungry.
In "Have Dragon Will Travel, Part 1", the Dragon Riders try to find an elusive Razorwhip and its rider, who are attacking ships. They use the Dragon Eye to bring up information about the Razorwhip. One bit of information is that this dragon loves to eat sea slugs. It forms the basis of their search - finding islands that have sea slugs. This is quite inaccurate, as those species classified as "sea slugs" are found in the sea, not on land.
In the episode, "Ruff Transition", sea slugs are seen in some detail as the Wingmaidens feed and bond with their new Razorwhip babies. Ruffnut must feed Wingnut but is having difficulties. Tuffnut grabs the sea slug from her, chews it up in his mouth, spits it into Ruffnut's mouth, who then spits it into Wingnut's mouth, who takes it easier.
One activity in the game is the "Journey". Ruffnut and Tuffnut have a Journey in which they "rule" an island that no more than a large rock that has sea slugs and Razorwhips on it. They dub it Thorstopia.
Hailfate feels most at home in the frigid valleys of the Tundra, where he carves deep banks into the snow while gliding over the glaciers. He has discovered that he can jab his long tail through the thick ice along the coast to hunt sea slugs. Vikings were quick to adopt this tactic for fishing, but their tools took much longer compared to the Razorwhip's sharp and powerful tail.[src]