There is no link between diet and excess mucus production. According to my GI and three of my primary doctors, excess mucus is usually a sign of an infection, and would have nothing to do with diet. There is also no link between dairy and mucus production.
You may actually have a case of excess phlegm, but it likely has nothing to do with the Soylent. You may be also be mistaking thickened saliva for a feeling of excess phlegm. Consider if you’re dehydrated (if you’re new to Soylent, you likely will be since we derive much of our water from food), if you have allergies, if you smoke, if you have acid reflux, or anything else before placing the blame with Soylent.
I’ve mistakenly attributed excess phlegm or a feeling of it with food in the past and my doctors have each corrected me numerous times over the years. Here’s some good reading on the subject:
Relationship between milk intake and mucus production challenged - http://www.ncbi.nlm.nih.gov/pubmed/2154152
The Milk Mucus Myth - http://www.healthyeating.org/Milk-Dairy/Milk-Myth-Busters/Article-Viewer/Article/29/milk-myth-2-drinking-milk-causes-mucus.aspx
Causes of Excess Phlegm - http://en.wikipedia.org/wiki/Phlegm
Causes of Thickened Saliva - http://buddymd.com/thick-saliva-sticks-throat