The app is installed on his phone, yes? Then he can't blame it on pop-up ads. He put it there and is using it to hide who he is talking to.
If that is enough for you to divorce, then you don't need to do anything else. If you are hoping to reconcile or need more before pulling the plug, then investigate rather than confronting. If you confront him right now it will let him know that he needs to hide the evidence, which makes it harder for you to figure out what is going on.