Add the water in a medium saucepan and sprinkle the agar agar powder over it. Stir to combine and set aside for 2- 3 minutes. Add half of the mango puree, sugar and lemon juice to the pan and bring to a simmer. Leave it simmering for about a minute, turn off heat then stir in the remaining mango puree. Set this aside and cool to room temperature. Meanwhile, whisk the whipping cream using a mixer until soft peaks form. Stir the vanilla extract to the cooled mango puree mixture. Fold half the whipped cream into the puree mixture, then fold the remaining half until mixture is uniform in color. Pour the mango mousse filling to the cooled tart crust. Smooth the top with an offset spatula. Chill the tart to set in the fridge for at least 4 hours.
Adapted from Food Network, recipe from Anna Olson.