The Acid Terrapus, characterized by slimy green skin and the ability to fire projectiles of acid, is a more dangerous breed of the Adult Terrapus. Acid terrapi are found at lower depths in close proximity to acid pools. They occur most abundantly in brain biomes. They will attack the player on sight.
Much like other Terrapi, the Acid Terrapus is a threat to your mining spree, so you'll have to either destroy it or run away from it. Your first option -- fight -- is probably the smartest.
Note that Acid Terrapi are immune to Acid Cannon attacks, so be sure to use a compatible weapon.
Your next option, running away, will be more successful if you dig twisting paths in the dirt. This will confuse the terrapus and allow you to make a quick escape.