This happens buddy its all good at least you opened your mouth and approached unlike most guys here's some things to think about, that will help your game.

Did you ever think before you even said it, how many times she probably has heard that before. That's like telling a gorgeous woman '' You're Beautiful '' instant wall goes up your just another guy tryna fuck, and in her case just another guy tryna fuck alexa spykids girl actor.
Why are you apologizing my bad, I hope you didnt put your head down to, either you asked because you really thought so or because you are being playful we want to come off as fun and playful and interesting not stupid don't apologize.

Now here's a toned up way you could have said and done the same exact thing.

You walk up to her with excitement and a smile on your face, you say '' Hey it's you no way can I have your autograph'' the whole time your smiling pretty much laughing being playful she can see it, she'll smile or laugh look around like what. you say '' yea arent you alexa blah blah from spykids'' the whole time your being playful smiling almost teasing. she'll be like blah blah no or she'll be like yea! what do you have to sign you got a pen(this is when you know shes a winner) most likely she'll be like blah blah no ive heard that blah blah basic chick blah blahness. Then you'll be real smooth and say I was just teasing you, transition right into more fun conversation for her have her laugh open her up get some convo find out logitistics from her , why she riding the bus do you two stay close by , etc take her on a instant date, then from there go back to your place the end.