A regular motion named "pulsing" was observed in this video. Stinging water is a common phenomenon that upside-down jellyfish made, in which it produces mucus in water namely "cassiosomes" which consists of an outer epithelial layer of nematocytes surrounding by endosymbiotic dinoflagellates hosted within amoebocytes and presumptive mesoglea. Those mucus are capable of killing prey and making discomfort to swimmers.
Close-up photo of flower-like oral arms of Cassiopea sp. jellyfish.