Forum:Super Polymerization ruling

Hello there! I run an E-Hero deck, and I use Super Polymerization in conjunction with Elemental Hero Gaia. At a recent tournament, my opponent had an Earth-type monster on the field, and then summoned Elemental Hero Prisma. When he summoned him, I used Super Polymerization, and he called priority on Elemental Hero Prisma effect. So thus, when he discarded his card, he changed the name of his monster so that it was no longer Elemental Hero Prisma. I was under the impression that Super Polymerization was spell speed 4, and i'd be able to get his Prisma, but the judge ruled against me. Was this a correct ruling? And if so, why or why not?