Is mentioning your ex-girlfriend to a potentially interested girl okay?

Of course. You're trying to drive her off, right?
Whaaaa . . .? You're interested in her? That's a shame since it's never going to happen now.
Seriously, when you can't avoid it without lying or inexplicably changing the subject, you should give contextually appropriate information about an ex. To as much as imply that one exists otherwise is to label yourself irredeemably non-datable.
You see, unless and until you have the new girl's unconditional trust, she will assume the only possible reason you could have an ex is because you're a horrible monster of a man who fiendishly and maliciously did all you could to break your poor set-upon ex's little pure heart.
So, bet the house this "potentially interested girl" you mentioned sees you that way now. Good luck with that.