Moss is a simple plant seen many times in the How to Train Your Dragon Franchise, usually in the background scenery.
Moss is a simple plant lacking vascular tissue - tissue to carry nutrients and water up and down the plant. Mosses are classified in the Division Bryophyta, separate from vascular plants. They lack flowers and instead reproduce by spores. Most moss species prefer a moist cool shady environment and can be found growing in tufts, sods, or mats on moist ground, tree trunks, rocks, and even rooftops. It's usually green in color, though it can also come in shades of brown, yellow, or red.
The major function moss is shown to have in the Franchise is rolled into little balls and used as earplugs, which Hiccup makes for their dragons so they won't be hypnotized by the songs of the Death Song in the episode "Tone Death". Later, an individual Buffalord is indicated to feed on moss.
In real life, moss has been used historically in northern latitudes as bedding, padding in clothing such as boots, as chinking between wood slats and log in buildings, as a water absorption material for items such as diapers, and used to aid cooking. Certain moss species have been dried and used as fuel. Very rarely, moss has been eaten in times of food scarcity. In present day, moss is most popular in decoration and gardening.
Moss is seen many times while Hiccup is out in the woods, either searching for or heading out to meet Toothless at Raven Point. Rocks and tree trunks are covered in green moss.
In "Tone Death", Hiccup gives the Riders ear plugs he says he made from moss, so their dragons won't be drawn in by the songs of the Death Song dragon when they travel to Melody Island in hopes that Garf will make himself at home there. They seem to work as none of the dragons are pulled off course by the songs of the wild dragon.
Mosses of various colors are seen in Valka's Mountain, coving the rocky surfaces.
Moss also appears in this game's iteration of Deathsong Island (another name for Melody Island)
A Buffalord exclusive to this game, named Mosster, is said to eat moss exclusively.
|“||This monstrous Dragon sustains its massive magnitude on moss alone. Nobody knows how it stays so big!||”|
|— Dragons: Rise of Berk|