While fighting a large group of enemies in Act 2 Normal, some of them used an ability that created a circle on the ground, with a sort of green, swirling vortex pattern in it. I didn't get a screenshot before they disappeared, and I haven't seen this effect since.
My first thought was "green = poison", but it didn't seem like I took any damage from standing in them.
What is this effect? Is it unique to a particular monster type? Or a random elite monster ability?