gracilaria verrucosa
蘺 refers to a specific type of red algae, Gracilaria verrucosa. Its meaning is a narrow botanical term with no broader conceptual center.
蘺 combines the grass radical 艹 with 離, which likely contributes the sound. The exact historical development is uncertain.
The grass radical 艹 sits above 離, like a specific seaweed (a grass-like water plant) named after its sound.
For リ, imagine a reed (a grass-like plant) swaying underwater: reed -> リ, and the seaweed 蘺 is a kind of water grass.