a dimorphous expression is when you express

A dimorphous expression is when you express something different than what you are feeling e.g.  when you cry when you’re happy or laugh when  you’re nervous.  It’s the body trying to regulate the emotion.
– WTF Fun Fact

Source: Frontiers | Anger and Sadness Expressions Situated in Both Positive and Negative Contexts: An Investigation in South Korea and the United States | Psychology (frontiersin.org)