黒慈姑

Literally, “black arrowhead”, 黒(くろ) (kuro, “black”) + 慈姑(くわい) (kuwai, “arrowhead”). The kuwai changes to guwai as an instance of rendaku (連濁).

黒(くろ)慈姑(ぐわい) or 黒慈姑(クログワイ) • (kuroguwai) 

1. Eleocharis kuroguwai, a kind of spikerush

As with many terms that name organisms, this term is often spelled in katakana, especially in biological contexts (where katakana is customary).