What is the fibrous lining in my concrete greywater basin?

I have a grey water sump pump in the basement of my house (1958, Dayton, OH) that also serves as a grey water basin for the basement utility sink, furnace condenser, and washing machine. It is a crude bowl shape made from concrete (probably dug out after the slab pour). In the 12 years I have lived here it has never filled with ground or rain water. Today while I was clearing a clog from the pump, I noted that it has been some years since I have completely drained the sump and wet vacc up any debris. The sides of the sump were looking like they had a buildup of old soap on them so I decided to use a paint scraper to clean it off some.

As I started to clean it out I found that the entire sump had a half inch, meshy, carpet like stuff going all the way around it. I assumed it was just a combination of too many years buildup of hair, lint, soap, and just other yuck and lightly scrapped it out. It was stuck to the sides, but with not much effort I was able to pull most of it up in 2 or 3 large chunks. The top edge of this did blend very well into the sides of the sump. Behind this thing, I also found some very small roots running along it, and a larger one. I was also able to make out a on-its-side brick in the lining of the sump.

So what I’m wondering is did I just stumble on a obsolete sump lining system and probably should have left it alone?