Chances are that it is not worth pursuing. Let's say that you are not instantly shut down and a sexual harassment charge is not levied against you, or you otherwise inappropriately express your feelings and are terminated. The chances are that the relationship fails and you or them have bitter feeling that interferes with work moving forward.
Could it be "the one?" Sure. I've know folks who got married who met at work, one was even the owner and an employee. I've seen amicable hook ups and flings. Is it worth the risk? Probably not.
Also, just as an FYI, expressing interest is not sexual harassment. Repeatedly doing so is, or doing so in an inappropriate way is. But, again, probably not worth it.
Edit: oh, HR might also have policies against it.
A lot of great love stories happens at work. Recently, my wife and I were listening to Obama's Audacity of Hope and Phil Knight's Shoe Dog. Obama pestered Michelle until she agreed to go out with him. Phil Knight hired Penny when she was a student in his class cause she was cute. Then went out with her while he was her boss. They both are married to same person still. My wife thought it was so cute.
But there is so much risk involved. Personally, I never did pursue anyone at work. It is probably because I care about my career.
Also initially you think that crush is the one and you will never find anyone else as perfect as them. But then after a few months when crush is gone, and you can see all the flaws in that person and you are glad you didn't risk your job and career for them.
But we were 20 and single at the time, so it depends on your circumstances, not much background from OP. Having a fling with a married co-worker probably ain't going to end well.
Why wouldn't you pursue someone if they show mutual interest? You just have to be more careful, more respectful, and use more tact than if you had met someone in a bar (though personally I'd adhere to these regardless of circumstance). If you're not sure how to approach it with these general principles, then it's probably best to stay out of it until you're confident enough in yourself.
You also have to realize that if it doesn't work out, it'll be harder to break things off cleanly.
But to rule it out altogether? That's a bit extreme.
Been there, done that. It's not good, especially in a workplace that has less than half a dozen people, and it's impossible to avoid seeing the other one all day long.
Having said that, there is the situation where the workplace is pretty large and the two of you are in (or can move to) different departments. That can work quite well because you don't meet or see each other every five minutes.
Been there, done that too. We're still together after 30 years.
Then I'd tell the girl.
If she's that important, finding job elsewhere shouldn't be a lot of work.