Even when the Gelatinous Cube is in plain sight, a creature must succeed on a DC 15 Wisdom ( perception) check to notice the cube if the creature hasn't witnessed the cube move or otherwise act.

Acid, Blinded, Charmed, Deafened, Exhaustion, Frightened, Prone

A creature with blindsight can perceive its surroundings without relying on sight, within a specific radius. Creatures without eyes, such as oozes, and creatures with echolocation or heightened senses, such as bats and true dragons, have this sense.

Range 60 ft

As an Ooze  , your body has a defense mechanism of an acid shield on your surface. You do 1d4 of acid damage to an attacking enemy who inflicts direct melee damage to you.

The Gelatinous Cube moves up to its Speed without provoking Opportunity Attacks. The cube can move through spaces of creatures the same size or smaller of it has room inside itself to contain them.

Room inside of cube is one creature the same size or three of smaller sizes.

Dexterity Saving Throw: DC 12, each creature whose space the cube enters for the first time during this move. 

Failure: 10 (3d6) Acid damage and the target is engulfed. An engulfed target is suffocating, can't cast spells with a Verbal component, has restrained condition, and takes 10 (3d6) Acid damage at the start of each of the Gelatinous Cube's turn. When the moves, the engulfed target moves with it. An engulfed target can try to escape by taking an action to make a DC Strength ( athletics) check. On a successful check, the target escapes and enters the nearest unoccupied space.

As an action other creatures within five feet can attempt to pull the engulfed target or objects out of the Gelatinous Cube by succeeding in DC 12 Strength (athletics) check and the puller takes 10 (3d6) Acid damage

Success: Half damage, and the target moves to the nearest unoccupied space within five feet of the cube. If there is no unoccupied space, the target fails the save instead.

Ooze size by level:

Tiny: level 1

Small: level 3

Medium: level 7

Large: 10

Huge: 15

Gargantuan: 20

Gelatinous Cubes are capable of halting their growth.

They can continue to consume but grow no larger than they desire.

Once a size has been obtained though the Gelatinous Cube is unable to revert to a smaller size.