Jelly Cube (5e Creature)
From D&D Wiki
Tiny ooze, unaligned
Ooze Cube. The cube takes up its entire space. Other creatures can enter the space, but a creature that enters the cube's space must succeed on a DC 10 Dexterity saving throw or take 5 (2d4) acid damage.
Transparent. Even when the cube is in plain sight, it takes a successful DC 15 Wisdom (Perception) check to spot a cube that has neither moved nor attacked. A creature that tries to enter the cube's space while unaware of the cube is surprised by the cube.
Pseudopod. Melee Weapon Attack: +3 to hit, reach 5 ft., one creature. Hit: 5 (2d4) acid damage.
Jelly cubes are small oozes that greatly resemble gelatinous cubes, and are thought to be closely related to them. Like gelatinous cubes, jelly cubes are all but transparent, and they feed primarily on small animals and insects that are too slow to escape them or unlucky enough to blunder into them.
Ooze Nature. A jelly cube doesn't require sleep.