Antennae

The rust monster corrodes a nonmagic metal object within 5 feet. It can destroy up to a 1-foot-square portion of an unattended object. If the object is worn or carried the objects owner makes a DC 11 Dexterity saving throw avoiding the rust monsters antennae on a success.

Metal shields or armor the rust monster touches with its antennae corrode, taking a permanent -1 penalty to its AC protection per hit

If the penalty reduces the armors AC protection to 10 the armor is destroyed. If a metal weapon is touched it is subject to the Rust Metal trait.

Bite

Melee Weapon Attack: +3 to hit, reach 5 ft., one target. Hit: 5 (1d8 + 1) piercing damage.