Kombucha is a fermented food, and in fermentation the cultures of probiotics like to form a cellulose matrix to live in (called a SCOBY). At Health-Ade, we try hard to filter that out before we bottle it, but sometimes the cultures can make it through. This is a totally natural part of the brew, and not indicative of anything foul or bad.
Many here consider it a “bonus” because it’s arguably a big dose of the good stuff. At the same time, if you don’t like it, just don’t drink that part. It’s like the “mother” in apple cider vinegar.