26F 26M how do I confront my boyfriend?

My boyfriend and I have been together for around 3 years at this point, we've had our ups and downs but we have been working on them together, and getting better at it each day.

We don't hide our phones from each other, we share the password on them, and have access to them.

Now... I was doing something on my phone and was going to do it on his too (for an app, which I did do btw), when I let the little nagging feeling in the back of my mind win and look at something I've been finding suspicious on his phone.

Turns out, said "suspicious thing" was nothing, HOWEVER, I found something else 🥲

A message from a previous "love interest/romantic-/sexual partner" well into our relationship. I've never heard of this name before, which made me even more uncomfortable, because we've spoken about some of our previous relationships before and so I know a few names or characteristics/ethnicity, but not with this one. She didn't match anything we spoke about before. ESPECIALLY because she was of a race he claims he "hates". I'm not gonna go into the details of that, but I think it's also good to mention, we have agreed to delete all previous people we have had an interest in or have slept with, on all socials. Which was an issue before actually, because it turned out at some point that he did not do that. I deleted them all, whilst he kept a fair few, and then after an argument and some back and forth, he finally agreed to get rid of "all", or so he claimed ..

I don't know what the message was because it's not been saved, but I also don't know how to ask him about it.

I don't want to have a misunderstanding, or an argument, but I've also been letting this eat me up for the past few hours and don't want to keep going. I also don't want him to try and turn the conversation around and on me for going on his phone (which even though we agreed is okay, I can see why it'd be upsetting considering he was asleep, originally I didn't want to wake him up just for the intended purpose, but then I let the nag win and the rest is history, or his-story?) lol, sorry, I'm trying to make it light hearted and laugh so I don't cry, I feel fairly betrayed right now, and don't know how to proceed.

Please if anyone can advise how to approach this conversation or situation as well as maybe giving me some pointers if he does try to turn this on me instead of talking about the message, thank you!

TLDR: found messages from a girl on my bfs phone, being sent well into our relationship, don't know how to confront/approach him about it.