you should get this cleaned off imediately and use either silicone grease applied or if that is not availble, at the bare minumium grease which is synethehtic based not petroleum like this one, this will swell up the rubber and cause the slides pin to stick which is dangerous.
MB sells brake paste it goes on the back of the pads when putting them in the carrier. helps to ensure they remain soundless. liqui moly anti squeal paste is also availble.