Seaweed Colcannon

Name: Seaweed Colcannon
Ingredients: 3 pounds russet potatoes, peeled and cut into large chunks; 2 tablespoons salt plus 1/2 teaspoon, divided; 1 cup lightly packed dulse seaweed; 5 tablespoons unsalted butter; 2 cups finely chopped cabbage; 4 scallions, minced; 1 cup whole milk