As Spotify looks to improve the overall Spotify experience, they’re now automatically generating usernames, hence the random letters and numbers.
You don’t need to remember this to log in. You can access your account with your email address and password.
During signup, you were able to personalize your account by creating a profile name that’s displayed in the app. There are no limitations to a profile name, so you were able to choose any name you like 🙂
Note: It's not possible to log in with the profile name.