Yes, mucoidal plaque exists. You know how you can find out? Do a Bowel Cleanse yourself. Not only will you see it coming out, but you will also notice a huge difference in how you feel.
I personally would do some research on quackwatch before you believe anything they say. I guarantee these people are not at all interested in helping people heal. Here is a link to get you started.
I'm curious, what do you need this "proof" for? Are you just curious, writing a book, interested because you are trying to solve your own health probems, what is the deal?