Heat a large pot over medium high and add oil. When oil shimmers, add white and light green scallion parts to pot and sautee until soft, about a minute.
Add garlic and ginger to pot and cook until fragrant, about 30 seconds.
Add clams, tomatoes, coconut milk and half of the corn to the pot and cover.
Clams should begin to open after about five minutes. As clams open, use tongs to pluck them from the pot and place into a large bowl. The timing often varies, but all the clams should be open after about 15 minutes of cooking.
Once all clams are cooked, add the remaining corn to the pot and cook for a minute. Remove from heat and stir in the lime juice.
Taste broth and add salt or additional lime juice if necessary.
To serve, ladle broth into individual bowls and add clams. Scatter herbs, scallion greens and chillis on top, if using.