Acid Form. The elemental can enter a hostile creature's space and stop there. It can move through a space as narrow as 1 inch wide without squeezing.

Multiattack. The elemental makes 2 acid whip attacks.

Acid Whip. Melee Weapon Attack: +6 to hit, reach 5 ft., one target. Hit: 13 (1d8 + 4) acid damage.

Acidic Whelm (Recharge 4–6). Each creature in the elemental's space must make a DC 15 Constitution saving throw. On a failure, a target takes 13 (1d8 + 4) acid damage. 

The elemental can grapple one Large creature or up to two Medium or smaller creatures at one time. At the start of each of the elemental's turns, each target grappled by it takes 13 (1d8 + 4) acid damage. A creature within 5 feet of the elemental can pull a creature or object out of it by taking an action to make a DC 13 Strength and succeeding but takes 1d6 acid damage by doing so. 

Oportunity Poison. Melee weapon attack: +5 to hit. Reach 5 ft., one target. Hit 13 (1d8+2) acid damage. The creature makes a Constitution 14 Saving throw, if failed the target is poisoned, taking 1d4 poision damage each for 2 turns. The target can make an additional Constitution saving throw to end the effect early on the turn of each crature, but takes 1 poison damage on each failed saving throw.