Pomacea bridgesii or apple snail, the white one is typically called the Ivory Snail, while the brown is more to nature with it's original coloring. They don't eat live plants unless no other food is found, but they can reproduce in multitude with an excess of food. Most of the time I find them eating algae off the tank and scouring the substrate for food. For a snail, they move pretty good, and they can grow up to 6 inches giving them the title of the largest fresh water snail.
Thanks for taking a look at the pictures,