1. Combine sugar, tapioca, milk, coconut milk and beaten egg in a medium saucepan. Stir, then let stand for 5 minutes.
2. Bring the tapioca mixture to a full, thickly bubbling boil over medium-high heat, stirring constantly. Reduce heat to medium-low and cook, stirring, for 2 minutes. Remove from the heat and let cool for a few minutes. Then whisk in lime zest, 1 tablespoon lime juice and 1 1/2 tablespoons creme de cacao.
3. Pour the tapioca into 4 fluted glasses or dessert dishes. Place plastic wrap directly on the surface to prevent a skin from forming and refrigerate until set, at least 2 hours or overnight.
4.To serve, whisk the remaining 1 tablespoon lime juice and 1/2 tablespoon creme de cacao in a small bowl. Spoon over each serving and top with coconut. Garnish with a lime slice, if desired.