Although it is a somewhat unusual pattern (most places still ask for both username/email and password at the same time), I don't think it need be particularly confusing for users (and, as Bohdan Karplyak's answer notes, Google currently do something like this, so they presumably don't think users would get confused!)
However, it may be worth considering why most sites have historically asked for both at the same time, and whether there are any on-going reasons to continue to do so.
Historic reasons to ask for both at the same time
One reason to ask for both at the same time would have been performance: especially in the past, every interaction with a web-server would have taken time, so collecting as much information as possible before contacting the web-server would have been a good idea. These days, this is generally not a concern.
The other main historic reason was security. If a user supplies both a username and password, and collectively they are not valid, security best practice was / is to not indicate which is valid / invalid. In other words, you don't have separate messages for "That user does not exist" and "That is the incorrect password" (implying that the user does exist). Responding like this would make an attacker's job (slightly) easier since they could first search for a valid username, then search for its password. Responding with the recommended "The username or password is invalid" means they have to get both right at the same time.
While this was certainly valid when accessing "traditional" computers (where accounts are created solely by system administrators), it is less relevant for websites where new users can more-or-less freely register. With such systems, it is quite difficult to not reveal information about whether a username exists (for example, by trying to create an account as a tentative user). There are ways to limit this, but usually at a cost to legitimate user-experience. Generally the potential security benefit is outweighed by real user inconvenience, so responding with "Create a new account" or "Welcome back, what's your password" isn't the problem it used to be.
Potential new reason to ask for both at the same time
Increased security awareness means users are increasingly being encouraged to not use easy-to-guess/remember passwords, and to not use the same password (or ones with small variation) for multiple websites. In most cases, this amounts to a push toward using password managers of one kind or another.
While password managers get ever better at detecting where to fill-in user credentials, and "best practice guidelines" for web-designers exist to make their job easier (see, for example, Making password managers play ball with your login form), some password managers may struggle if the username / email is entered on one screen, and the password on another.
Presumably (hopefully?), if the split-entry model is followed by more and more websites, password managers will get better at coping with them (and/or best practices for web-designers will be updated to allow password managers to cope).
From a pure user-experience point-of-view, there's probably no reason not to use a "split-entry" design. However, from a security point-of-view, it is probably advisable to check how well a number of the most popular password managers cope with your site. If you cannot get them to work, users will be discouraged from using "good" passwords.