Found discord on my 13yo's phone, now what

Wasn't snooping just grabbing his phone to check the time and saw discord notifications popping up. I didn't read the messages but clearly he's been hiding this from us. He knows the rules about social media. Part of me wants to let it go because all his friends are probably on there but another part of me feels like he violated the trust we built. Should I confront him or does that just make him hide it better? He's a good kid, good grades, not giving us problems. This feel like a real issue or am I overreacting as a parent?