Algae are photosynthetic, eukaryotic organisms classified as protista.
Kelp buds are a type of Human seafood. They are produced by kelp, an algae, which can be found in Earth's oceans and is sometimes informally referred to as "seaweed" by Humans. (TNG: "The Wounded")
The uniforms worn by Starfleet personnel in the 23rd century were made from xenylon, a material which was made from processing a type of algae. Since the Starfleet uniforms were synthesized from an organic form of matter, they contracted along with the crew exposed to the shrinking ray. (TAS: "The Terratin Incident")
In 2269, the crew of the USS Enterprise was on a mission to the planet Argo, where they had to collect seawater and algae specimens. (TAS: "The Ambergris Element")
Algae is among the sea foliage the Aquans harvest for what they term as "edibles". (TAS: "The Ambergris Element")
In 2373, Odo and Mora Pol planned to place algae near an undeveloped Changeling so it could learn to transform into it. (DS9: "The Begotten")
Slug-o-Cola is a Ferengi beverage that contains 43% live algae. (DS9: "Profit and Lace")
The Breen fed Ezri Dax and Worf a diet of algae paste while they were being held as prisoners aboard a Breen warship. (DS9: "'Til Death Do Us Part")