To answer your question: Yes, you can!
But, really, I believe this might be more than changing that green notice. Here's what I would do and why (this depends on who your target audience is and what your team is willing to build, so pick and choose as you please; any of the following will help):
- Remove asterisk and "denotes required entry" from the login page. You need a login phrase (pin, username, or email) and a matching password to enter mostly any website account on the Internet.
- Change "email" to "Email." The Username and the Email will now both have equal focus and there will be more consistency across the site (you have "Confirm Password" and "Email Address", both words in caps, in the registration page).
Change "Email Address" to "Email" to keep consistency with the Login page.
Try to keep all textbox names immediately next to, whether on top or on the side, of the corresponding textboxes. This would mean that if you still want to have any notices, keep them below the textbox or on the opposite side. This will make it easier for a user to tell what a current textbox is for.
"Only 50 characters allowed". If you want to keep this phrase, consider removing "only," so that it wouldn't say "haha, this is all you get!" An email address above 50 characters long is usually an extreme case, so I would have it be a message that appears only if such an email address is entered and the "Create Account" button is pressed or you could have it say dynamically "/chars_left_variable/ characters left" in a subtle gray under the textbox.
The green notice... ideally would be removed. An elegant solution to resolving some of the confusion could be adding a placeholder, the contents of which would depend on the target audience. So the email placeholder could be "firstname.lastname@example.org" and the username placeholder could be "johny". Something else that might help is adding a line break between the email and the password sections, making it more awkward for someone to enter all of the information the same in one section (the "answer-cant-be-C-five-times-in-a-row" trick).
Using a backslash in a password/username can also be considered a relatively rare instance (who are they trying to hack, eh?), so let's remove the "any character allowed except for \" notice and add it as an error when pressing "Create Account". As for the 5 character limit, if you really want to, you can remove that notice as well and have a green checkmark when that is satisfied.
So why do all of this?
Consider the information and decision overload that a user might go through in a day. Wear this, eat that, do this, help him, address her. At the end (or beginning) of the day, the user just wants to go through this routine task without having anything to think about. By removing the above-stated information, you basically make it mentally hurt for the user to even think about writing the email in the username field - they can clearly see that, because they don't have to process the extra 35 words that were clouding their decision-making or whether the Username textbox is above or below the "Username" line.