hello jackson,
that's actually the current behaviour. if you enable "allow email login" the message will say "your email" and when you disable it then you see the "email or username" text. however if you want to change the text to "your username" only you can do this by using a language override. the string that needs to be altered is COM_EASYSOCIAL_LOGIN_USERNAME_PLACEHOLDER

hope that helps but if you have any questions feel free to get back to us at any time. have a nice day