Bring water and lemongrass to boiling in a small saucepan. Remove from heat; cover and let stand until cool, about 30 to 45 minutes. Strain, reserving liquid; discard lemongrass. (Add water, if necessary, to liquid to equal 1/2 cup.) Return liquid to saucepan. Sprinkle with gelatin; let stand 5 minutes to soften. Cook and stir over low heat just until gelatin dissolves; cool slightly.
Combine cream cheese, sour cream, cilantro, garlic, chili pepper, and salt in a food processor bowl or blender container; add gelatin mixture. Cover and process or blend until well-combined. Transfer to a medium mixing bowl; stir in the 1-1/2 cups shrimp.
Line a 7x3-1/2x3-inch loaf pan or a 3-1/2- to 4-cup bowl or mold with plastic wrap. Spread shrimp mixture evenly in container. Cover and chill for 4 to 24 hours. Unmold onto a serving platter; remove plastic wrap. If desired, garnish with additional cooked shrimp and fresh cilantro sprigs. Serve with assorted crackers. Makes 12 servings.