<% if current_user %>
  <p>You are signed in as <%= link_to current_user.username, user_resources_path(current_user) %>!</p>
<% end %>

<% if current_user %>
  <p>You are signed in as <%= link_to current_user.username, user_path(current_user) %>!</p>
<% end %>

Change resource_path to user_path for username link.
Change resource_path to user_path for username link.
