If the inside surface of the pipe has a coating of grease in the pipe and the water is still flowing, I wouldn’t worry about it.........If you are determined to get it out you can try sodium hydroxide in some form. There are many drain cleaning products available, like Draino etc. Draino is a mixture of sodium hydroxide and other chemicals that produce heat when mixed with water. Grease and soap are similar except soap is soluble in water. Soap is made from mixing grease and sodium hydroxide also known as lye or caustic soda. There is a product called Red Devil Lye. It is mixed with water and if heated is very effective. I have used it to clean floors, remove paint and to clean drains. It will eat aluminum and produce hydrogen which can be explosive. Sodium Hydroxide can be a dangerous chemical and can cause burns to your skin and if it gets in your eyes it can cause blindness, so you need proper safety equipment. Most degreasers contain sodium hydroxide as their main ingredient.