Heat oil in a 6-qt. saucepan over medium-high heat. Cook garlic, chiles, ginger, and onion until golden, 4–6 minutes. Add masala and tomatoes; cook, stirring occasionally, until tomatoes begin to break down, 5–7 minutes.
Add coconut milk, salt, and 1⁄4 cup water; boil. Add mussels; cook covered, and occasionally shaking pan, until all mussels are opened, about 5 minutes. Garnish with cilantro; serve with rice, if you like.