I hope there is a snail expert looking, because my best advice is to do nothing unless you plan on setting up an aquarium specifically for snails. Anything you do to make the snail happier may do the opposite to the fish. Wait a minute; I did think of one thing. Mystery snails lay their eggs outside the water. If your aquarium has a glass top, they need a couple inches of air between the water surface and glass top. There is a small chance the snail could be gravid with eggs, or maybe he/she needs another snail for company. Finally, you could try putting an algae wafer near the snail; most of them need their vegies. That was two or three things, wasn't it?