The Merfolk is a non-binary term for mermaids and mermen it is also used for sea beings in general (especially hybrids or mermaid/merman-like).
Etymology[]
Mere is Old-English for sea. Maid is a word in old and modern English meaning either a maiden or a maidservant. Man is both old and modern English for either a human or an adult male. Folk is a word from modern English meaning people.