Should I talk to my SO if I think he's cheating?

I was sort of wondering what counts as cheating and if I should address something that came up with my fiancé. The other day he asked me to get his phone for him, and he doesn't have a passcode on it, so I happened to open it and noticed that it was on this app called MeetMe, which is a dating app. I'm not familiar with the app, but it looks like he was talking to other women on it. I didn't say anything and I didn't snoop further because we are pretty honest with each other and I trust him, and I simply gave him back his phone. But now I wonder if I should say something and if this counts as cheating if he has been talking to other people. I'm a little afraid to address it though. I have extreme anxiety and depression and he has has helped me through so much. I feel like it would be cruel to accuse him of this when he is really sweet and I trust him. I don't know what to do and I've been thinking about it lately and I worry he might be cheating. Should I talk to him or should I just give him the benefit of the doubt? And if I did talk to him how would I bring it up without sounding like I was accusing him right off the bat?

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