You want to hold onto your audience. You should display no reaction to settlement offers Because no one really knows what the true settlement value of a case is before the settlement agreement is actually reached, you should not display any reaction to any settlement offer, either verbally or nonverbally. If your opponent makes what she thinks is a reasonable offer to settle, and you scoff at it, your reaction might anger the other side sufficiently to cause them to withdraw from the mediation. If you are a defendant, impulse reactions could do similar damage to the potential for successful negotiations. Verbal reactions are not the only type of prohibited reactions to settlement offers.
Facial expressions can communicate what people are thinking.
In the proper context, they can be a strong asset. In the wrong context, they can be a devastating liability. You should never argue Refraining from arguing is a cardinal rule everyone should observe at mediation. Arguing carries two possible connotations. That is okay.