Theoretically, this process can go on indefinitely, effectively rendering the jellyfish biologically immortal,[3][6] although in practice individuals can still die. In nature, most Turritopsis dohrnii are likely to succumb to predation or disease in the medusa stage without reverting to the polyp form.[7] --WP
Yeah, that works for fiction. In the real world though, nothing can survive entering a black hole for example. So your definition of immortal is pretty useless outside of fiction.