Rinse the mussels and tap any open ones on the counter. Discard those that do not close within a minute, and any with cracked shells.
In a large pot, combine the water, ginger, and scallions and bring to a boil. Add the mussels, cover, and cook for 4 to 6 minutes, shaking the pot once, until the shells open wide. Lift out the opened mussels as they appear and discard any that stay shut.
Strain the liquid through a fine sieve lined with a paper towel to remove grit. Shell the mussels, discarding the shells, and cover the meat to keep it moist.
Rinse the pot. Combine the rice, 7 cups water, strained mussel liquid, and 2 ginger slices. Bring to a boil, then lower to a gentle simmer with the lid slightly ajar.
Cook for about 45 minutes, stirring every 5 to 8 minutes, until the grains have burst and the congee looks creamy and thick enough to coat the back of a spoon. Add hot water if it thickens too much.
Meanwhile, chop the parsley and capers finely, or pulse them with the garlic in a food processor. Stir in the lemon zest, lemon juice, vinegar, and olive oil, and season with salt and pepper.
Taste the congee and season with salt and pepper. Remove the ginger slices. Stir in the mussel meat and let it warm for 1 minute off the heat.
Ladle into warm bowls. Spoon over the salsa verde and top with microgreens. Serve immediately.