Preheat oven to 350°F. Line an 8x8-inch baking pan with parchment paper, leaving overhang on two sides. Grease lightly.
Toast the pecans: Spread chopped pecans on a small baking sheet and toast in the oven for 5 minutes, or in a dry skillet over medium heat for 3-4 minutes, stirring until fragrant. Set aside to cool.
Shred and squeeze the zucchini: Grate zucchini on the large holes of a box grater. Place in a clean kitchen towel, gather corners, and twist tightly over the sink to squeeze out as much liquid as possible. Measure about ½ cup of dry shreds.
Mix dry ingredients: In a medium bowl, whisk together flour, baking soda, salt, and cinnamon.
Mix wet ingredients: In a large bowl, whisk melted butter and brown sugar until smooth. Add egg and vanilla, whisk until combined.
Combine: Add dry ingredients to wet, folding with a rubber spatula until just barely combined (a few streaks of flour are fine). Fold in squeezed zucchini and toasted pecans, reserving a handful for topping. Do not overmix.
Spread batter evenly into prepared pan. Sprinkle reserved pecans on top. Bake for 22-26 minutes, until edges are golden and a toothpick inserted in the center comes out with a few moist crumbs (not wet batter). Let cool in pan on a wire rack for 15 minutes, then lift out using parchment and cool completely on rack (about 30 minutes).
For the glaze: Whisk together powdered sugar, milk, vanilla, and a pinch of salt until smooth. Drizzle over cooled bars. Let set 10 minutes before slicing into 12 bars.