Curse of Rust
From Diablo Wiki
Curse of Rust is a spell cast by some monsters that halves the Armor value of opponents (v2.05). There are two ways to see if you are affected by this spell. First, there is a little icon above the "Achievements" icon in the menu bar that appears. Hovering over this icon tells you how many seconds are left before the spell terminates. Second, when you are under this spell, there are little red bubbles appearing above your head. One of the monsters that uses this spell is Aspect of Lies.