It is a rather small island, with a stream or river that stretches through its middle. There are several sea stacks littered around the island. There are presumably lots of algae or glowing algae in the area. It could possibly have a swampy and murky coverage at some areas due to the fact that the algae passes through it and because the Flightmare likes wetlands.
Algae Island appeared on this online map of the Barbaric Archipelago. However, no additional details were given about it on the map, nor did it appear in the Dragons: Race to the Edge series.