That "stuff" is called chaff. It is small flakes of cholesterol that has not formed into stones. Many people have a lot of chaff and it congests the biliary ducts as much as stones do. Some of my flushes have been almost all chaff with only a few stones, but, it better out than in. Skeptical or not, you had a good flush. Your liver will thank you.
By the way, some people will release chaff for several days, long after the oil and lemon pulp would have left the body.