What are the grey-black specks that appear when whipping cream in stainless steel bowl?
Those look like air pockets - you're using an unusual whisk, perhaps it can't get enough "bite" on the stainless steel bowl to pop them, whereas the plastic bowl's texture offers enough resistance. I'd try it with a balloon whisk rather than a spiral whisk, and see if that helps. Here's a breakdown on whisks and their uses from Craftsy.