It's a silhouette of a face without most important features, with exception to the mouth. Well often when people say "do the talking", stereotypical go to is to do it with your mouth.
However we are in the pixel realm wouldn't need it. Eyes would be gateway to your soul or mind. Left that unknown for whomever to discover if the chat is to progress further.
Just a play on social/physical norms for a non-physical realm.