Whisk together sweet potato casserole, milk, butter, and eggs in a large bowl. Whisk together flour, brown sugar, baking powder, cinnamon, and salt in a separate bowl. Gradually stir flour mixture into sweet potato casserole mixture just until dry ingredients are moistened.
Pour about 1/4 cup batter for each pancake onto a hot, lightly greased griddle or large nonstick skillet over medium. Cook until tops are covered with bubbles and edges look dry and cooked, 2 to 3 minutes. Turn and cook until puffed and cooked through, about 2 minutes. (Cook times will vary. If starting with a very wet sweet potato casserole, increase cook time by 2 minutes.)
Serve pancakes immediately with syrup and toasted pecans, or place in a single layer on a baking sheet, and keep warm in a 200°F oven up to 30 minutes.