I've had the same thing happen with severe constipation such as you describe. It's only normal bowel mucous and nothing to worry about by itself. You're doing a great job with making your own smoothies to address the problem. Definitely mention the constipation to your doctor. I can't imagine how uncomfortable you must be!
i just posted a similar question. did you ever find out what it was?
mention it to your dr - if it is white it would be worth checking your liver function - just to be on the safe side!
all the best vix