Worms may be living in and on top of the sewage crust, which means that the crust is old. If the tank were working properly, the crust would be reforming all the time. The bacteria live in the water immediately under the sewage crust. You can’t see them as they are microscopic and millions will fit on the sharp point of a pin. The fact that the worms are there means that the tank has not been working in biological balance, or has been subjected to sewage overload.