Partner/baby's father removing messages

In my apartment complex, we have a shared laundry room, so I need to do laundry outside my unit. I have the app on my phone to make payments. When he was taking my phone to pay for the laundry, I asked if I could use his phone to scroll through TikToks in the meantime. Initially, he handed it to me, but then he returned and said, "You know what? Show me how to get the app on my phone." I told him that was unnecessary since I already have money in my account. I suggested he use my phone for the payment while I used his for TikTok. He hesitated, saying he didn’t want me to change anything on his page and make different posts appear. I reassured him that I wouldn’t like or interact with anything, just scroll. He eventually relented and let me use his phone. Out of curiosity, I glanced at his messages and noticed he was texting two girls: one was his best friend, with whom we’ve had issues, and the other I didn’t recognize. I didn’t read the texts but noted the names. After returning to scrolling on TikTok, I felt the need to bring it up, so when he got back, I asked to see his messages. He questioned why, and I replied it was just out of curiosity. He told me to wait because he was busy with his phone and his game. I then noticed him quickly swiping and clicking, and when I looked again, the messages were gone. I handed back his phone and simply said, “Okay.” He asked, “What do you mean, okay?” and I repeated, “Just okay.” I want to address this calmly but feel it’s important to talk about what I saw. How should I approach this? I'm [23F] and he’s [22M].