Question about mold on the inside of a drain pipe

Has anyone ever seen this
“Periodically in one of the bathroom sinks, I need to remove the drain stop and clean out a fungus type of sludge growing on the plastic stem and attached to the wall of the pipe. It is black/brown/white in color and has a skin to it.”
What would cause this. I have a friend of a friend asking me.
I actually have this in my house also.
Andy B

All of the appropriate environmental conditions are present for this sludge to grow, is it not?
Temperature, moisture, oxygen, and a food source.

It could also just be crud that collects from soap or anything else you use the sink for. In my own house mine does this just a little bit, but my wife’s side is always really bad.

My reasoning… she uses more crap than I do. About the only thing my sink is used for is brushing teeth and washing hands. Hers is used for the same, but then throw in all the different hair products that end up on her hands or in the sink, makeup, makeup remover, lotion being washed off her hands, etc. it just seems like it sticks to the plastic parts and then gets funky over time.

Andrew…exactly what i thought. I have a double sink and the wifes side is the worst. I was thinking yes, the right conditions and all the product that goes down it.