Pseudopod

Melee Weapon Attack: +5 to hit, reach 5 ft., one target. Hit: 10 (3d6) acid damage plus 9 (2d6 + 2) bludgeoning damage.

Engulf

The cube moves up to its Speed. While doing so it can enter Large or smaller creatures spaces. Whenever the cube enters a creatures space the creature makes a DC 13 Dexterity saving throw. If the creature is unaware of the cubes presence it makes its saving throw against Engulf with disadvantage. On a success the creature may use its reaction if available to move up to half its Speed without provoking opportunity attacks. If the creature doesnt move it is engulfed by the cube.

A creature engulfed by the cube takes 10 (3d6) acid damage plus 9 (2d6 + 2) bludgeoning damage, can't breathe, is restrained, and takes 10 (3d6) acid damage plus 9 (2d6 + 2) bludgeoning damage at the start of each of the cubes turns

It can be seen but has total cover. It moves with the cube. The cube can hold as many creatures as fit in its space without squeezing.

An engulfed creature can escape by using an action to make a DC 13 Strength check

On a success the creature moves to a space within 5 feet of the cube. A creature within 5 feet can take the same action to free an engulfed creature but takes 10 (3d6) acid damage plus 9 (2d6 + 2) bludgeoning damage in the process.