If you accidentally saw your boss watching something inappropriate on his personal phone at work, would you say anything?

My boss is male, mid to late 30s. He's married and has kids. I'm in my late 20s, married with kids. I've known him for about 5 years. We have nothing going on between us. I'm several rungs below him on the corporate ladder.

Today (Friday) I went to drop off some documents for him in his office. While he was looking at the documents some movement caught my eye and I saw maybe 2-3 seconds of something on his phone. It was long enough to realize it was two people having sex. ***Clarification: he had his phone beside him on his desk facing up when he was working facing the door. Then I walked in. He took the documents from me and turned around to scan something on his scanner. He left his phone facing up between us but now his back was turned***

I feel so icky about it but it's his personal phone. He does not know I saw it because his back was to me when I caught the movement out of the corner of my eye.

Do I say anything to HR? On the one hand, it's none of my business. Maybe his wife sent him a dirty video, although I doubt it because this was a pretty explicit video with two people in it (if you know what I mean).

No matter what I think about doing, it feels wrong. Telling HR or talking to him about it feels invasive and like I'm somehow violating his privacy. Not saying anything feels like he gets away with watching porn at work which is not ok, but it's also none of my business what he does on his personal phone.

Yes, I am 100% sure of what I saw. What should I do? What would you do?

Edit: His office door is always open and he works facing the door. He knew I was coming in because I called before I came by. Now that I think about it, it just makes me feel icky. I think I'm overthinking it.

Realist: Yes. He turned around after I handed the documents to him. He was facing me when I came in though and he had to scan something from what I brought him. I had to wait for him to hand me back the originals. His scanner is behind him so he had to turn around (back to me) while he was scanning the documents.