Combine the ginger, garlic, coconut, water, onions and salt in a large pot. Bring to a boil, and then reduce to a simmer and add the octopus. Cook for 10 minutes, and then transfer to a bowl to cool. Meanwhile, mix together the mirin, rice wine vinegar, toasted sesame oil, soy sauce, sriracha and lime juice in a medium bowl.
Once the octopus is cool enough to handle, slice the thicker tentacles into 1/2-inch slices, and then leave the thinner, curly ends long. Mix together the octopus, chiles, cucumber, and green onions, and then toss with the dressing. Serve at room temperature or cold in the papaya halves.