That white residue is the tell-tale fluid that MB puts into their coolant for just the purpose that you have experienced. It is a tell-tale for leaking components in your cooling system. So consider yourself lucky that they do this, because otherwise you might never have known that you were leaking.
I would concur strongly that you attend to this/these leaks as soon as possible. "Frying" an engine due to loss of coolant is never inexpensive (double negative intended)! You are talking major bucks (new head, new pistons, new valves, etc. etc.)....Andras