Gray Ooze

The ooze can move through a space as narrow as 1 inch wide without squeezing.

Any nonmagical weapon made of metal that hits the ooze corrodes. After dealing damage, the weapon takes a permanent and cumulative −1 penalty to damage rolls. If its penalty drops to −5, the weapon is destroyed. Nonmagical ammunition made of metal that hits the ooze is destroyed after dealing damage.
The ooze can eat through 2-inch-thick, nonmagical metal in 1 round.

While the ooze remains motionless, it is indistinguishable from an oily pool or wet rock.

Melee Weapon Attack +3 to hit, reach 5 ft., one target. 4 (1d6 + 1) Bludgeoning damage plus 7 (2d6) Acid damage, and if the target is wearing nonmagical metal armor, its armor is partly corroded and takes a permanent and cumulative −1 penalty to the AC it offers. The armor is destroyed if the penalty reduces its AC to 10.