Acid Trail. The Acid Snail leaves a 10ft. wide trail of acidic slime in its wake lasting up to five minutes that is difficult terrain for other creatures. A creature takes 2d8 acid damage the first time it enters the trail on its turn or starts its turn in it.

Acidic Body. When the Snail takes damage of any type but psychic, each creature within 5 feet of the Snail takes 9 (2d8) acid damage.

Multiattack. The Rock Snails makes up to two attacks with each of it's Tendrils, uses Shell Defense or Acid Secretion.

Tendril Melee Weapon Attack: +5 to hit, reach 10 ft., 1 target. Hit: 6 (1d4 + 4)  bludgeoning damage and 9 (2d8) acid damage. 

Shell Defense. The Acid Snail withdraws into its shell, gaining a +4 bonus to AC until it emerges. It can emerge from its shell as a bonus action on its turn.

Acid Secretion (Recharges after a Short or Long Rest). The Acid Snail secrets a large amount of acidic slime in a 10ft. radius around its body. Each creature must succeed on a DC14 Constitution Saving Throw or take 4d8 acid damage - half of it on success.