Grease like substance on radiator cap

What is your question?

I noticed a grease like substance in the rubber gasket of my radiator cap. Unable to determine what it was i decided to drain the cooling system and refill with a flush product. I ran the vehicle for 4 days with product in water, drained filled with water only twice. There was probably 100 miles in all driven during this time. I refilled with 50/50 mix prestone and have since driven the vehicle another 100 miles. I checked the radiator for full since I didn't see any drop in level from overflow tank. To my surprise there is more grease like stuff than before. What could this be?