Shapechanger. The mimic can use its action to polymorph into a wall or back into its true, amorphous form. Its statistics are the same in each form. Any equipment it is wearing or carrying isn't transformed. It reverts to its true form if it dies.

Adhesive (Object Form Only). The mimic adheres to anything that touches it. A creature adhered to the mimic is also grappled by it (escape DC 16). Ability checks made to escape this grapple have disadvantage.

False Appearance (Object Form Only). While the mimic remains motionless, it is indistinguishable from an ordinary wall.

Grappler. The mimic has advantage on attack rolls against any creature grappled by it.

Pseudopod. Melee Weapon Attack: +8 to hit, reach 15ft., one target. Hit 16 (2d10 + 5) bludgeoning damage. If the mimic is in object form, the target is subjected to its Adhesive trait, and the target must make a DC 16 Strength saving throw.  On a failure, the mimic pulls the target up to 10 feet closer to itself.

Bite. Melee Weapon Attack: +8 to hit, reach 5ft., one target. Hit 16 (2d10 + 5) piercing damage plus 11 (2d10) acid damage. If the target is a Medium or smaller creature, it must succeed on a DC 16 Strength or Dexterity saving throw (target's choice) or be swallowed by the mimic. A swallowed creature is blinded and restrained, has total cover against attacks and other effects outside the mimic, and takes 14 (4d6) acid damage at the start of each of the mimic's turns.

If the mimic takes 20 damage or more on a single turn from a creature inside it, the mimic must succeed on a DC 21 Constitution saving throw at the end of that turn or regurgitate all swallowed creatures, which fall prone in a space within 5 feet of the mimic. If the mimic dies, a swallowed creature is no longer restrained by it and can escape from the corpse by using 20 feet of movement, exiting prone.

Mortar (Recharge 5–6). The mimic spews brick mortar in a 15-foot cone. Each creature in that area must make a DC 16 Dexterity saving throw.  On a failed save, the target is restrained. It must then make a Constitution saving throw at the end of each of its turns. If it successfully saves three times, the effect ends. If it fails its save three times, it encased in hardened cement and is subjected to the petrified condition. The successes and failures don't need to be consecutive; keep track of both until the target collects three of a kind. 

A creature petrified by this effect can be broken free of its cement casing by another creature, removing the petrified condition.  The cement has AC 17, automatically fails saving throws, and 15 hit points.