Yes, that's preserved lemon in the gremolata. It's a salty lemon pickle, common in Morocco and the Indian subcontinent. While you can make your own, it takes weeks. You can find small jars of them at specialty markets or stores like Whole Foods.
Rinse your oysters thoroughly in cold water. Pat dry with paper towels. Place cup side up on a hot grill until they begin to open.
While the oysters cook, finely chop your parsley, preserved lemon, and garlic. Add to a bowl, stirring in olive oil, lemon juice, and salt to taste.
Once oysters have begun to open, remove with tongs to a flat surface; a rimmed baking sheet works well. Pry each oyster fully open with an oyster shucking knife, discarding any that look dry, shriveled, or have a bad smell.
Arrange oysters on serving tray, topping each with 1-2 teaspoons gremolata. Serve immediately.