>i dont know what my next move should be..
if you see her while she's cosplaying:
"Hey, I know this anime character you're cosplaying as!"
else:
"Hey, you were cosplaying as [character] the other day, right?"

that's my opinion. I think I would be too shy to do that if it happened in real life (on top of the fact that I don't have a waifu)