Looks like mud ? What type coolant have you added ? Take the radiator cap off an look into it , do you see the same ? Have you ever had the cooling system flushed ? There was a problem with people adding green coolant to GM - Dex-cool ,orange ! Clog's the passages in radiator ,inside the engine , heater cores etc... Pull the oil fill cap on the valve cover an look at the under side of the cap , do you see brown frothy looking goop ? Coolant getting into the oil ! Why it called DexKILL