I agree too that it is a member of the family Pelagiidae, probably Pelagia sp. but I I am not so sure that it is Pelagia noctiluca. Probably ask André C. Morandinii (is on research gate) - he is an expert for Pelagiidae.
I fully agree with Ilka, this is not P. noctiluca. It might be P. panopyra, a poorly known taxon surely worth of further investigation. I attach herein a pic of it, taken at the Amakusa marine lab, Japan. I am available to barcode it in my lab, if you have preserved specimens in ethanol. It would be great to collect and store at least one or more voucher specimens in formaldehyde 4% in seawater, and fragments of oral arms, gonads, bell margin, from 1-5 specimens (separately from each individual) in ethanol 70-90 % for DNA analysis. Send me your email address at [email protected] and I will send you an easy protocol for sampling and storage. All the best, Stefano