That is an ivory mystery snail. They love to eat duckweed--which is the only live plant I've seen them munching on. Mystery snails come in:
ivory (light food, white shell),
gold (light foot with golden yellow shell),
chestnut (light foot and brownish shell),
black (dark foot, dark shell, wild type),
blue (white shell with a dark, bluish foot),
magenta (light foot, pinkish/purplish striped shell) and
purple (dark bluish foot with pinkish/purplish shell).
I think that's all of them. Ivory, gold, and chestnut are very easy to find in the trade. Blue, magenta, and purple are more difficult to find.