Re: Saw Mario Williams at 24 hr
I will give Mario the benefit of the doubt that perhaps he was lost in thought and didn't hear the OP. Otherwise, I think if a person recognizes someone, make eye contact and chooses to say hi, whether the person is a celebrity or any other acquaintance, a response in kind would be common courtesy.
It appears the OP said the greeting in passing and if Mario did indeed hear the greating, it would be impolite to ignore the greeting. It would be different if the individual attempted to start a conversation or tried to get an autograph that perhaps Mario would elect to decline.
So far, the celebs that I have had a brief exchange have been cordial.