It looks to me like a type of exhaust joint sealant. Wurth perhaps. Some 00 steel wool or a grey Scotch Brite pad with plenty of elbow grease should take it off. As Bush said fine 500-800 grit sandpaper could sand it off. A bronze wire brush on a die grinder would be my preferred way of getting it off. A bench grinder with a bronze wire wheel is one of my favorite tools for cleaning nuts, bolts axles etc.