In a large saucepan, combine the five grains, watermelon rind, and 6 cups of spring water. Bring to a boil. Cover and reduce heat to low, and simmer until the liquid is reduced to 4 cups, about 50 minutes.
In a small saucepan, combine the rock sugar, tea bags, and 1 1/2 cups of spring water. Bring to a boil. Reduce heat to medium-low, then simmer for 2 minutes, then remove tea bags. Continue to simmer the mixture until the rock sugar dissolves.
Discard rind. Ladle soup into serving bowls, flavor with sugar and green-tea mixture. Serve warm or cold.