In a nonstick skillet over medium low heat, combine grated ube and condensed milk. Cook, stirring regularly, for about 8 to 10 minutes or until thickened.
Add powdered milk and lime zest, stirring to combine. Continue to cook until mixture can be pulled away from pan in one soft ball. Remove from heat and allow to cool.
Using a teaspoon, divide mixture and shape each portion into logs of about 1-inch long. Roll pastillas in sugar until fully coated. Wrap each piece in Japanese paper or cellophane.