Glowing Algae are a unique type of algae that glows bioluminescence as part of their metabolism. The algae is able to survive in both freshwater and saltwater as it typically flows through rivers and occasionally or eventually flows out to sea. The algae attracts Flightmares, which feeds on it as its natural food source. The algae gives the Flightmare a much brighter glow which adds to its unnatural appearance. The Glowing Algae is possibly plant but could also be a type of dinoflagellate, such as other glowing micro-organisms. More so, Dragon Tracker Part 3 states that the algae stream is also fish-filled.
The Glowing Algae glows bright blue and appears as droplets of glowing blue blobs when separated from its stream. They appear to be sticky when also separated from its stream which carries more water, as it was able to stick to the side of a ship and not fall. As well as sticking to Ruffnut's and Tuffnut's faces for a while.
The Glowing Alga's glow was thought to be a reaction to Aurvandil's Fire, which is a type of Aurora Borealis or Northern Lights display, as mentioned by Hiccup upon first discovery of the algae. It was also stated that it glows bio-luminescence in the light of Aurvendil's Fire. However, this theory was false as when, a Glowing Algae stream was seen at night without the presence of the aurora, meaning that the algae produces its glow by its own natural metabolism.
As being the main part of the Flightmare's diet, the Glowing Algae apparently contains all the nutrients to sustain the Flightmare's life and as well as sufficient energy for the Flightmare's abilities. The Flightmare is attracted to the algae because it makes the Flightmare able to glow much brighter than its self-produced glow as a side effect. Therefore, the Flightmare is able to control how bright it glows with this added effect. However, this effect is not exclusively to Flightmares. Other dragons can eat/drink this algae and also glow in their natural color. But although it can make other dragons glow, other dragons do not have the ability to control their brightness.
The Glowing Algae also seems to have a strong effect on the Flightamre's shot limit. According to Dragonpedia, the Flightmare has a shot limit of 4, which is significantly low and inaccurate, when the Flightmare produced a long trail of its toxic mist to surround the Dragon Riders. It is impossible for the Flightmare's original shot limit to be 4 for it to produce such a long trail of its mist without any signs of regeneration. The only explainable reason is that the Glowing Algae has another side effect that increases the shot limit of the Flightmare, or even the toxicity in the mist, since they are both the same color and is somewhat "sticky" as the Flightmare's mist can create a "shiny" aspect on its paralyzed victim.
- It is possible that the Glowing Algae is somewhat related and/or based on other bioluminescent micro-organisms in the group of dinoflagellates such as Noctiluca scintillans (or otherwise known as Sea Sparkle) which is a type of marine dinoflagellate. There are many similarities between the two. The Sea Sparkle's glow is a result of a reaction when they are "disturbed". Although, the Glowing Alga's glow is proven to be not caused by any reaction. However, both glow very bright at night and are somewhat the same color. Noctiluca scintillans is considered toxic to the water around it, and the Glowing Algae, having an effect on the Flightmare's shot limit, is also possible to the direct cause of the toxicity of the dragon's firepower. They also share a similar appearance, however the Glowing Algae has to lose water, and the Sea Sparkle is required to be closely packed in order to share resemblance of the "blob" appearance.