That's a tadpole, or bladder snail. As long as you don'r overfeed, they shouldn't breed too much, but they can get out of hand. They don't really damage plants, they tend to eat rotting vegetation rather than healthy stuff.
You can either squash them, 'bait' them overnight with a piece of lettuce that you remove in the morning, or leave them be. If the population does get out of control, you can think about adding some assassin snails which will eat them.