I want to tell my stepdaughter that I would prefer if she did not call me dad
That's interesting, why don't you want her to call you dad?
I don't want to mislead her into thinking that I'm her real dad.
I am aware of your concerns, but it's already quite an achievement that she is ready to call you dad. You should be proud of that, as many others would with that happen. You need to let your fears go and let her call you that, because genes don't dictate who you can have as family.
But she has a dad, and it's not me. I'm just married to her mom, that doesn't make me her father and I don't want to be financially responsible for her.
Blended families can be difficult to manage. Have you and your wife talked about your parenting responsibilities, if not, it's probably time to do so? It seems like she wants you to be involved as a parent, but that's not what you want.