Local dialect. It is only because of the unwritten rule that "katakana is used for biological names" that it is written in katakana, which is a foreign word at first glance. Since it is a local word, there is generally no reason to call it that, but there are two interesting stories about the origin of the name "クラゲ". The ancients thought that jellyfish had no eyes, which led to the sound of "くら" related to "darkness"; Judging from the way jellyfish drift with the current, it is associated with the mimetic word のらりくらり ("laziness").