You know what it looks like… but what is it called?
TAKE THE QUIZ“mermaid's”
The following 2 entries include the term mermaid's.
mermaid's-hair
noun: a common filamentous marine blue-green alga (Lyngbya majuscula) growing in long matted tufts on eelgrass and on larger algae
mermaid's purse
noun: the dark horny or leathery egg case of various skates or other elasmobranch fishes
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Merriam-Webster unabridged