Heat oil in a large pan and add the mustard seeds. When they start to pop add asafetida powder, onion, ginger & garlic. Fry until the onion is golden. Then add the turmeric powder, green chilies and curry leaves and fry for couple of minutes. Pour the coconut milk in this mixture and salt to taste. When it starts simmering, add the shrimps and mix well. Cook uncovered for 15 minutes or until the shrimps are cooked. Serve hot.