The literature would suggest that snails will begin climbing upwards on the glass when water quality is poor. Based on my own personal experience, I am not too convinced of this. I don't have MTS but pond and some other types of ugly snails(I am unsure of type) that do this all the time. I don't believe this indicates a water quality issue. In my case, if there was a water quality issue, my Amano Shrimp and Otocat, which are the most sensitive to water quality would be dead.No one knows what it means when the snails come out to the open from the substrate?
In your case, I would suggest testing your aquarium water just to be safe. Many aquarium shops test the water for free. If your ammonia levels are fine, I would not worry too much about it. The only other thing that snails need is sufficient calcium. I have read about calcium deficiencies will cause snails to die amass. Their shells turn all white, indicating a calcium deficiency or in some cases too low a PH.