Why did Leo abruptly call stargirl susan?

Leo abruptly called Stargirl "Susan" as a way to address her by her real name, which represented a more conventional identity in contrast to her unique and eccentric persona as Stargirl. This moment highlighted the tension between his feelings for her and the pressures of fitting in with societal norms. It was a reflection of Leo's internal conflict and the societal expectations that often lead individuals to conform at the expense of their authenticity.