Drain tapioca pearls. Combine reserved coconut milk and coconut milk from remaining can in a medium saucepan; add salt and ⅓ cup agave. Bring to a boil over medium, reduce heat, and simmer, stirring constantly, until reduced by one-quarter, about 5 minutes. Add tapioca and cook, stirring constantly, until tapioca is translucent and tender but still chewy in the center, about 5 minutes. Set saucepan in a bowl of ice water and let cool, stirring occasionally.