Algae were photosynthetic, eukaryotic organisms classified as protista.
Kelp buds were a type of Human seafood. They were produced by kelp, an algae, which was found in Earth's oceans and which was 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 2373 during their attempt to teach an infant Changeling how to shapeshift, Doctor Mora suggested that he and Odo expose the Changeling to simple lifeforms such as algae, fungi, and then invertebrates. (DS9: "The Begotten")