The Green Slime is a Slime enemy that spawns during the Day. The Green Slime drops Slime Knife and Slime Chunk. The Green Slime has 5 HP, and deals 1 damage. The Green Slime can spawn above ground during the Day, but does not spawn underground. They slide across the ground when neutral, and once a Green Slime sees you, it will slide faster towards you.
(Image from the Terraria Wiki)