If you made an appointment and the girl you asked for greeted you - you should stay or leave a tip if you leave, if the girl you made an appointment with is not available, feel free to leave and say you’ll be back another day when she is available, if so simply walk in and she’s not available, you can walk out without any issue….
Always be nice, you might be surprised what kindness brings, even if she is not a attractive as the girl you wanted to meet….but your choice to leave as well….