It's normal-- the yeast nutrient is causing the CO2 in the mead to come out of solution. It's not a chemical reaction, just physical in that the nutrient is providing nucleation points for the CO2.
On Deck: Jamil's Vanilla Robust Porter
Fermenting: Orange Blossom Mead
Kegs: Element 56 Pale Ale, Ron's Belgian Blonde, Summer'n Saison, Furloughktoberfest '09, Grateful Pale Ale, Sam Adams Cream Stout Clone, EdWort's Apfelwein