Place syrup in a small saucepan; bring to a boil. Reduce heat; simmer, uncovered, 5 minutes. Transfer to a bowl; cool completely.
In a small heavy skillet, cook and stir pecans over medium heat, about 3 minutes. Drizzle with 1 tablespoon cooked syrup; cook and stir 1 minute longer. Spread on foil to cool.
In a small bowl, beat cream until soft peaks form. In a large bowl, beat ricotta and mascarpone cheeses until light and fluffy. Gradually beat in 1/3 cup cooled syrup; gently fold in whipped cream.
To serve, spoon mousse into dessert dishes. Drizzle with remaining cooled syrup; top with candied pecans.