Yes this is tarnish. I use Wright's Silver Cream on my mouthpieces and it works great. Yes keeping them in pouches or wrapped in the original small plastic bags they ship in inside of the boxes can help. Leaving them out exposes them to a fresh supply of airborn pollutants (mostly sulpher compounds) that causes tarnish. I was my daily used mouthpieces with handsoap and water just before I practice almost every day and that also keeps it from accumulating tarnish.
Doug
Adams E3 0.60 Sterling bell - Prototype top sprung valves
Concord Band
Winchendon Winds
Townsend Military Band