Dairy is mucus forming, and whey is most definitely mucus forming. It's not that protein in itself is intrinsically mucus-forming but that the items commonly associated with high protein are mucus forming. I think possibly a complex amino acid supplement would be preferable for protein supplementation at least during a cleanse.
Plus, if you eat mucus-forming foods during the cleanse, the active ingredient such as cayenne is only eating at the fresh mucus instead of putting its full force into attacking the old mucus. Mucus is the response of the immune system to toxic food, trying to protect the body from it. Eating something that causes that response shows the body is trying to fend off toxins, not extract and eject them.
This is just my idea.