The one that looks like someone washed their hair in the rock pool has filaments that are thin, green, and silky, like strands of wet hair. They grow in dense mats, covering rocks, shells, and other seaweeds in a green, slimy blanket. You can run your fingers through it, feel the silky texture, the way it clings to your skin. It is the alga of the hairy surface, the one that makes the rock pool look like a shower drain. What makes it special is the simplicity. The hair algae is a simple green alga, a single, branching filament of cells, each one connected end to end. It has no complex structures, no floats, no specialised parts. It just grows, branching and branching, forming a tangled mat of green threads. Under a microscope, the filaments are beautiful, each cell a perfect cylinder, the branches forming at precise angles. The hair algae is a master of colonisation. It can grow on almost any surface, rock, sand, mud, shell, plastic, metal. It can tolerate a wide range of salinities, from fresh water to full-strength sea water. It can grow in bright sun or deep shade. It is the weed of the underwater world, the one that grows where others cannot, the one that turns bare surfaces into green carpets in a matter of weeks. Biologically, the hair algae reproduces by fragmentation, a piece broken off by a wave can grow into a whole new plant. It also reproduces by releasing spores and by simple cell division. This is why it can appear so quickly in a new area. A single fragment can colonise a whole rock pool within weeks. It is the alga of the quick spread, the one that never misses an opportunity. To find hair algae is to find the green slime on the rock. It looks like wet hair, feels like wet silk, clings to everything it touches. It is not beautiful. It is not dramatic. It is just there, covering the rocks, growing in the sun, being hairy. It is the alga of the rock pool, the one that most people ignore, the one that proves that the most successful organisms are not always the most glamorous.