Can you post a picture of the grub like creature? It is probably a fluke or a piece of a fluke. Quite possibly a liver fluke. They can be round if you expel them undigested or they can look like parts of mushrooms if you just get parts.
The bottom line is that you may never find out what it is. As long as you are finding them in the toilet, you are doing something right. Keep it up, for months after you finish finding things. You have to kill them thru their entire life cycle, which can be months.