Place water in a small bowl and sprinkle gelatin over top. Whisk to combine and set aside.
In a medium bowl, beat egg yolks and sugar until thick and smooth, about 2 minutes of beating. Heat milk in a medium saucepan over medium heat until steaming. (do not let boil) Very slowly drizzle hot milk into egg yolk mixture, a little at a time to temper the egg yolks. Once all the milk has been added to egg yolks, add all back to saucepan. Cook over medium heat, stirring occasionally until mixture is slightly thickened and at a low simmer.
Offheat, strain hot milk mixture through a fine mesh sieve into a medium bowl. Whisk gelatin mixture, white chocolate, and vanilla into milk mixture until chocolate and gelatin is melted. Let cool completely.
In a separate medium bowl with a handheld electric mixer OR stand mixer, beat heavy cream to stiff peaks. Very gently fold whipped cream and strawberries into cooled white chocolate mixture.
Spread filling over cooled crust and smooth top. Cover pan with plastic wrap and refrigerate mousse bars 2 hours until set.