Heat oil in 5- to 6-quart Dutch oven over medium-high heat. Add pepper; cook, stirring, until almost tender, 4 minutes. Add scallion whites, 1/4 cup cilantro, garlic, and pepper flakes. Continue to cook, stirring, until fragrant, 30-60 seconds.
Add tomatoes and coconut milk; bring to a simmer. Reduce heat to medium; simmer to blend flavors and thicken sauce slightly, about 5 minutes.
Add shrimp, partially cover and cook, stirring often, until just cooked through, about 5 minutes more.
Stir in lime juice and adjust seasonings, adding salt, if necessary. Serve shrimp stew over rice. Garnish with scallion greens and remaining cilantro.