For glaze, in a small bowl, combine brown sugar and mustard. Gradually whisk in the peach or apricot nectar until smooth. To prevent ham from curling, make shallow cuts around the edge at 1-inch intervals. Brush one side of the ham slice with glaze.
Grill ham, glazed-side down, on the greased rack of an uncovered grill directly over medium-hot coals for 6 minutes. Turn ham. Top with peaches and peppers. Brush ham, peaches, and peppers with glaze. Grill for 6 to 10 minutes more or until heated through, brushing occasionally with glaze.