Moss

Moss is a tiny, flowerless plant seen many times in the film franchise, usually in the background scenery.

Description
Moss is of the Musci class, reproducing by spores and growing in tufts, sods, or mats on moist ground, tree trunks, rocks, and rooftops. Its soft texture makes it popular as bedding for animals, and even humans if it's not too damp. It's usually green in color, though it can also come in shades of brown, yellow, or red.

Function
The only real function it's 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". In real life however it is eaten by many species of animals and sometimes humans as well.

How to Train Your Dragon
Miss is seen many times while Hiccup is out in the woods, either searching for or heading out to meet Toothless. Rocks and tree trunks are covered in green moss.

Dragons: Race to the Edge
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.

In "Living on the Edge", the twins are using the moss ear plugs while they sleep, and are thankful at how well they work at keeping the unsteady volcano fem waking them up.

In the episodes "A Matter of Perspective" and "Searching for Oswald... and Chicken", moss is seen covering the stony backs of the Sentinel dragons that guard the Island of Vanaheim.

How to Train Your Dragon 2
Moss of various colors is seen in Valka's Mountain.