Put the couscous in a medium heatproof bowl; add 1 cup boiling water and stir. Cover with plastic wrap and set aside.
Heat the olive oil in a large pot or Dutch oven over medium-high heat. Add the onion and cook, stirring often, until it starts softening, about 5 minutes. Stir in the paprika, cumin, 1 teaspoon salt and a few grinds of pepper. Cook, stirring, until the onion is coated, about 1 minute. Stir in 1/2 cup water, the tomatoes, green beans, olives and brine, and cilantro. Bring to a simmer and cook until slightly reduced, about 5 minutes.
Stir in the mussels. Cover and cook until they start opening, 3 to 5 minutes. Stir in the shrimp; continue to cook, covered, until the mussels are open and the shrimp are opaque, about 5 minutes. (Discard any unopened mussels.) Fluff the couscous with a fork. Divide among bowls; top with the seafood stew and more cilantro.