Whilst he may be a star in the MA/MMA community, this does not exactly mean he is driven round in a limo, or being pestered for autographs in the supermarket- he is just another guy to the vast majority of the world, and as such shouldnt have any approachabilty problems IMO.
Just talk to the guy, if you train in the same gym, and you compete then you are equalls. Get over this mental block, what if you end up having to fight him in a couple of years? what you gonna do, freak out and not hit him out of respect?! He is a guy, he will either be pleasant or a jerk, say high and find out