I have bought a couple of these snails from a local pet store, and I'm wondering if I chose a wrong kind.

They are a little larger than an inch in diameter, and looks brownish beige. The problem is, these snails have huge appetite. They seem to like the same type of food that my Cherry shrimps eat. They can swallow the entire fish flake with one bite! A piece of zucchini that previously lasted a week now lasts only 1/2 a day. They nibble on my plants! My Mynos have lost all the bottom branches. Sagittaria doesn't look that healthy either. And I think they also like to nibble on the Java moss.
When I bought them I was expecting them to clean up the tank with algae. But I don't think they find the algae that tasty. As you can see in the picture there are patches of green spots on the glass, but the snails just seem to ignore them.
Can anyone recognize the snail on the picture? Should I be getting MTS or Briggs instead?