I have never owned a Mystery Snail, but about a month ago, my Nerite Snail was experiencing a similar problem. It had algae on its shell, wouldn't move, and wouldn't eat. Here is what I think.
1. Snails will probably need minerals in the tank, but your GH looks good. However, you might want to consider raising the pH, because I am pretty sure that snails will like higher pH, GH, and KH. Don't do this immediately, though. Double check with research or another forum member. What other fish do you have?
2. He might still be stressed about the move, I don't know about Mystery, but Nerite snails' schedules can be a little different when they settle down in a new environment, and they might go completely lethargic for a few weeks.
3. Do you have a lot of algae in your tank, or barely any? One of the causes for snail inactivity is that they don't have enough food. Even if you do offer other foods, snails usually prefer natural algae, rather than processed algae.
4. I can't back up this idea, but maybe the algae might be slowing him down. Happened to my Nerite, and I tried to get it off. It worked, and made him move faster, but stressed him out a little, so don't try it.
5. If your snail is reluctant to come off the glass, that is a good sign that he is still alive and strong, because I am pretty sure they will lose their grip when they die.