Add the mussels, octopus, and prawns and boil for 1 minute. The prawns should redden and mussels should start to open. Add the squid and the crayfish and continue to cook over high heat until the squid is just translucent, about 1 minute. Drain everything and place the seafood all together in a warm bowl. Add the scallions, vinegar, olive oil, peppers, salt and pepper, and the mint, and toss to coat. Add the greens, toss again, and serve warm with lemon wedges and rock salt.