Like attachment, under the avatar, it seems mail address. I want showing Username. For Solution, each users have to go to Account Page, after change Display Name. Can we provide automatic the changes? So, all of users display name will be Username.
Please note under the avatar display the Display Name not the mail address you mention. So users just need to fill the correct data(the date what he/she want to display) when he or she registers.
Mor info in this FAQ topic: https://wpforo.com/community/faq/nickname-vs-username-vs-display-names/