I just did a quick search on this and found a BikeRumor article on this very topic with info from Stan himself:
Why is there liquid seeping through the sidewalls of my tire after having it sit for awhile, or long after adding the initial sealant? Is it bad? What should I do to avoid/fix it? Does it matter what sealant or tire is used in this case?
Stan’s No Tubes: This starts with the fact that tires, even modern tubeless or tubeless ready, are not perfectly air tight. The liquid you can see seeping through sidewalls or occasionally old puncture sites is generally one or more of the ingredients used as a preservative or “longevity enhancer.” For the majority of sealants currently on the market, that ingredient is often some form of glycol or glycerine (clear liquids, oily in nature) as they typically blend well with natural or synthetic latex and are hygroscopic in nature. As the latex or coagulant begins to dry, the remaining elements continue to find tiny pores or imperfections in the tires that leads to the seeping look.
Basically no, it’s not bad and won’t hurt anything. It’s not a great look we’ll admit, and we are continuously looking at our own formulations as well as those of the competition. In the end we have a few goals – first, sealant has to seal, bottom line, second, we are looking to balance longevity with puncture protection. Sealant that doesn’t have decent longevity and user friendly characteristics is a painful product to work with and if you don’t have good puncture protection, see point 1.
You’ll find, if you haven’t already, that many sealants on the market exhibit some type of seeping just as you’ll find that some tire makers or some types of tire construction are more or less susceptible.
We know, there’s no such thing as a stupid question. But there are some questions you might not want to ask your local shop or…