My guess is that you have the Break Shields plugin active. Check if it's on in the Plugin Manager. If so, turn it off and test again and see if the icon is gone.
The icon itself represents how many "Shields" a battler has. Shields decrease when a battler takes a hit from an element they're weak to. When the shield count reaches zero, the battler is affected by a state defined in the Break Shields plugin's Parameters (by default the Stun state in the Database).