
She never said she was Mielle, she gave the Ros... Something name (the family name). The Prince assumed it must be Mielle since Aria (the other child) came from the slums. It wouldn't be unusual for him to know her name. She is well known through bad rumors and she is now part of a big noble family.

Well in the chapter where they run away together (from the fireworks) he told her her he knew who she was and what she was hiding (he was thinking of Mielle) but she assumed he was referring to the rumors about her, so it wouldn't be odd for her to think he knows who she is.
Also, she forgot she was trying to hide who she was to Asher, cause it had been a while. She wasn't trying to blend her identity with Mielle anymore.
Someone explains to me plz
Aria told him shevwas mielle at first that what cause the misunderstanding
Then he knew she was aria through rain
But when he met her at the cafe he called her aria before she knew he was rain's master and she nvr told him her name
Why didn't she react to that?