The “jellyfish that have returned” are the offspring of the ones sent up; they “hate life on Earth” because they were born into a world without gravity - no direction, different kinds of pressure, so when they return they have trouble adapting and (according to the Read More in the source) “Jellyfish babies, at least, have to deal with massive vertigo on Earth after spending their first few days in space,” which you can tell because you can measure how disoriented a jellyfish is compared with norma behavior. TL;DR your cthulu is an infant with a migraine