Looks like it might be beer stone. How long have you been using it between cleanings? Beer can add a lot of "debris" to components. You should clean between kegs using a commercial bee line cleaner (like BLC or equivalent).
It kind of looks like residue from the plating process, specifically the acids, ( in the spout), and where the spout meets the (body?), looks like plating over an "un-polished" weld.