Bittersweet, with the richness of Grand Marnier balancing the bitter liqueur and lemon juice. (I'm guilty of rebalancing this cocktail and adding the prosecco.)
This occurs due to the lemon juice not being kept properly or using a lemon juice which is based on real lemons... like ReaLemon brand. You will have lemon particles within these types of lemon juice and because this is being mixed with prosecco, it gets 'cloudy' or 'particle-ish'. (Couldn't define the right word, but you get what I mean.)
Anonymous