I'd politely tell them that I love their music and to enjoy their flight. Then I'd put on my ear buds and like @kpopandkimchi I would then internally scream for 5 hours. I'd also be praying they'd want to talk to me.
In that situation, if I ended up making eye contact I'd try to smile in a friendly manner briefly before looking away but mostly I'd spend the whole flight super awkwardly and obviously trying to avoid looking or in any way disturbing them.
I would think he looked kind of familiar lol but it probably wouldn't hit me until later. I like to think I'd be cool and strike up a fun conversation but I'm sure I'd make it completely awkward because I'm completely awkward lol