Stir together 2 tablespoons cold water and chia seeds in a small bowl; let soak at room temperature until water is absorbed, at least 20 minutes or up to 12 hours. Set aside.

Whisk together 3 tablespoons rose syrup, agar-agar powder, and 1 1/2 cups cold water in a small saucepan until well combined. Bring to a boil over medium, whisking constantly. Cook, whisking constantly, 1 minute. Pour mixture into an 8 1/2- x 4 1/2-inch loaf pan. Use a spoon to skim off any bubbles from surface. Let cool completely at room temperature, uncovered and undisturbed, until set, about 45 minutes. Run a knife along edges of pan. Flip pan over; gently tap bottom to release jelly. Cut into 36 (1/2- to 3/4-inch) cubes. Set aside.

Bring remaining 4 cups cold water to a boil in a medium saucepan over medium-high. Stir in salt, and return to a boil. Add vermicelli; cook, stirring occasionally, until al dente, 1 to 2 minutes. Drain and rinse under cold water; drain well, and set aside. Whisk together cold milk and remaining 3 tablespoons rose syrup in a medium measuring cup with a spout; set aside.

Evenly layer 1 tablespoon chia seeds and 2 tablespoons vermicelli into 2 (12-ounce) serving glasses. Top each glass with about 2/3 cup jelly cubes. (Reserve remaining jelly cubes for snacking.) Gently pour 3/4 cup milk mixture into each glass; top each with 1/2 cup ice cream. Sprinkle each with pistachios and rose petals.