Hi everyone

Assuming that I'm using a noun to specifically describe an action, which a barber does to clean hair fragments off your hair after your haircut is done, why do I have to use "hair washing" instead of "washing hair". According to the concept of gerund,"Hair washing" means that hair washes or hair is washing, which makes no sense to me; however, "hair washing" is unquestionably a formal noun. but why?

Let me make it more clear with an example sentence. "Hair washing is the cosmetic act of keeping hair clean by washing it with shampoo or other detergent products and water." cited from Wikipedia

Besides the above question, how can I determine what word can be used in which situation?

Thanks for answering in advance