I hesitate to call it a collective noun, but the term for a spontaneous grouping of jellyfish is bloom, (as in an algal 'bloom').
"Bloom" is usually used for a large group of jellyfish that gather in a small area, but may also have a time component, referring to seasonal increases, or numbers beyond what was expected. Another collective name for a group of jellyfish is a smack, although this term is not commonly used by scientists who study jellyfish. Jellyfish are "bloomy" by nature of their life cycles, being produced by their benthic polyps usually in the spring when sunshine and plankton increase, so they appear rather suddenly and often in large numbers, even when an ecosystem is in balance. (-- Wikipedia)
So in your sentence, I reckon you can use the same term ("the bloomy jellyfish").
I'm unaware of a verb to bloom in this sense, but on this basis you may feel able to coin it.