Jenn, mousseline is the name that Rose gave to her buttercream which is—correct me if I’m wrong, folks—basically an Italian meringue buttercream. There are also Swiss and French versions of meringue buttercreams from other sources.
I have a formula from baking school for a light butter cream that my textbook calls Creme Mousseline, but it’s nothing like Rose’s recipe. Uses whole eggs and egg yolks and a different mixing method. Actually, it’s more like her Silk Meringue Buttercream. Never made it, so can’t say if it’s any good. But perhaps Rose used the name “mousseline” to convey just that idea, of how light her mousseline is for a buttercream. The classic French buttercreams made with all yolks gave bc a reputation for being heavy. And the so-called buttercreams made with emulsified shortening and icing sugar ...well, no comment.