Apparently Mercedes call it "glide mode" or "gliding mode" depending which bit of marketing you look at. I couldn't find a technical description though (beyond what you already said).
I've seen it called "sailing" (and the icon on the dashboard is of a sailboat). See https://www.youtube.com/watch?v=S4N8BlXro9w (see 48 seconds in) - but this could be a language difference between territories.
Apparently Mercedes call it "glide mode" or "gliding mode" depending which bit of marketing you look at. I couldn't find a technical description though (beyond what you already said).