What do you do about a crush?

Six years and he's still around.

Girl ya better "crush" back b4 he moves on/away!

Follow your feelings. If your are not really friends, then there's no friendship to risk. And as long as he is a decent guy, the worst thing that can happen is that he will eventually turn you down. But he will still be civil about it. There is nothing wrong with pursuing a crush, and it's sometimes the beginning of a beautiful relationship.
as for what to do specifically, if you ever end up staying late for work together or something, ask him if he wants to grab dinner together before going home. Chat with him casually at work when you get a change. Overall, just approach him. And you'll get a definite reaction back if he likes you the same way.

Asking him to hang out sometime doesn't necessarily you mean like him. Mention to him, you know, what we have known each other 6 years yet I feel like I barely know you! Now you are working here. Seems like a good opportunity to 'catch-up' let's catch lunch when you are done work/let's go on a break catch some ice cream or something. Just play it cool and calm & really the goal should be to get to know each other.