That looks like a speech bubble inside the white circle (see the small tail at the bottom), so might be a message/SMS app - there are an awful lot of those, so I can't say which.
If that is what it is then in itself it tells you nothing: many people use apps other than the one that came installed on the phone (I never use the built-in app as there is always a better alternative in my opinion).
But it may be something else, this isn't a positive ID. If you really want to know you should ask the person whose screenshot it came from (as that's usually what these questions are about).