In a small saucepan, bring the grapefruit juice, 2 peel strips and the sugar to a boil, stirring, until the sugar is dissolved and the liquid is reduced by half, 8 minutes. Remove from the heat.
In a medium bowl, whisk the egg yolks, then whisk in half of the hot grapefruit syrup, pouring in a slow, steady stream. Whisk the yolks into the remaining grapefruit syrup and cook, whisking, over low heat just until thickened, 2 to 3 minutes. Immediately strain into a small bowl set in a larger bowl of ice water and let cool. Discard the peel strips.
In a medium bowl, stir the marshmallow cream until smooth, then fold into the grapefruit mixture.
Using an electric mixer, whip the heavy cream and honey on high speed until soft peaks form, 2 to 3 minutes. Fold the whipped cream into the grapefruit-marshmallow mixture.
Spoon the mousse into dessert cups and refrigerate for 2 hours. Cut the remaining 2 peel strips into slivers and sprinkle on the mousse before serving.