Jellyfish are cnidarians, a phylum which also includes corals and sea anemones. Jellyfish are a specific lifecycle stage of medusozoans (their own subphylum), while corals and sea anemones are anthozoans (another subphylum of Cnidaria).

So yup, they are polyps before sexual maturity. And you’ll find they’re more closely related to corals than they are to members of any other phylum.

Edit: I highly recommend that anyone who wants to get lost in the world of marine invertebrates check out WoRMS and then follow along in parallel on Wikipedia as you navigate the tree.