Non-Newtonian Fluid: When in normal circumstances the Thixotropic Slime is in a more "fluid" state when it gets hit by melee it becomes more of a semi-solid object. The rules for AC are semi-flipped for the Thixotropic Slime. When hiting the Slime one wants to hit lower then the AC to deal full damge, if the hit is 5 over it only deals 75% of the damage, if its 10 over it deals 50% of the damage, if its 15+ over it only deals 25% of the damage. Crits will "lower" the Ac by 5 till a long rest to recover (Ac of 10 becomes an Ac of 15 after a Crit).
Amorphous: The Thixotropic Slime can move through a space as narrow as 1 inch wide without squeezing
Transparent: Even when the Thixotropic Slime is in plain sight, it takes a successful DC 15 Wisdom (Perception) check to spot the Slime that has neither moved nor attacked. A creature that tries to move into the Slime's space while unaware of the Slime is surprised by the Slime.