Preheat the oven to 350°. Spread the pecans on a rimmed baking sheet and bake for about 8 minutes, until fragrant. Let cool completely.
In a food processor, pulse the pecans, sugar, nutmeg and salt until the nuts are finely chopped. Add the butter and pulse to incorporate. Press the crumbs into a 9-inch springform pan in an even layer. Refrigerate until firm.
In a medium saucepan, bring the maple syrup to a boil. Simmer over moderate heat until slightly reduced and a candy thermometer inserted in the syrup registers 235°, about 8 minutes; be careful it doesn't boil over.
In a standing electric mixer fitted with the whisk, beat the egg whites with the cream of tartar and salt until firm peaks form. With the mixer at medium speed, drizzle the hot syrup onto the whites and beat until incorporated. Increase the speed to high and beat the meringue until cooled to room temperature, about 5 minutes.
In a large bowl, beat the heavy cream with the vanilla until soft peaks form. Fold the meringue into the whipped cream until blended. Scrape the mousse into the crust and spread evenly. Freeze until firm, at least 8 hours but preferably overnight. (The mousse will not freeze solid like ice cream.)
Put the cranberries in a large heatproof bowl. In a small saucepan, combine the sugar and water and bring to a simmer, stirring to dissolve the sugar. Pour the syrup over the cranberries and cover the bowl with plastic wrap. Set the bowl over a saucepan of simmering water and cook undisturbed over very low heat until the syrup is rosy and the cranberries are tender but not broken down, about 45 minutes. Remove the bowl from the saucepan and let the cranberries cool completely. Refrigerate until chilled.
Remove the ring from the springform pan. Cut the mousse pie into wedges and transfer to plates. Spoon the candied cranberries over the pie and serve.