I talked to a plumber this week and never heard of it and I'm starting to realize that it just slime and mold. A few years ago I remodeled the bathroom and I did clean the drain quite well. What happens is as soap and body oils go down the drain, it starts to coat the top part of the drain flange and pipe. Over time this scum starts to mold up and lines the pipe. It is so even that you swear it was sprayed on. I saw the same thing at my sons house and the house was maybe a year old at the time. He stopped using it because it looked like it was a seal coating and there could be a leak. If anyone that has a walk in shower you should pull the drain cover anyway to clean it out because a lot of hair does build up and it is a source of odors.