The reason why Disk Password is showing up as a user is because you formatted your volume as APFS (Encrypted). To do this, your volume had to be encrypted by a password. When you installed macOS onto the volume, FileVault recognized that the volume was already encrypted and that only the disk password you set earlier could unlock it. Hence, the disk password became a user on the FileVault unlock screen. Later, when you added users to the volume, FileVault used their passwords as additional keys to the volume and added them as users to the FileVault unlock screen. Now, both your disk password that you set earlier and the users on that volume can unlock FileVault. If you log in using Disk Password, you'll get sent to the login screen since Disk Password isn't a real user.
If you format your volume as APFS (not encrypted), and then later use FileVault to encrypt your volume, FileVault will nondestructively convert your volume to APFS (Encrypted) and only use user passwords (and the recovery key) as keys that can unlock the volume. Turning off FileVault will revert your volume to APFS (not encrypted).
To remove Disk Password, log in as an administrator and enter sudo fdesetup list in Terminal. You'll be prompted for your password (Terminal will not show characters while you type your password). Enter your password and hit Return. Terminal will list all users that are authorized to unlock FileVault. Then, use one of these commands to remove the Disk Password user:
sudo fdesetup remove -user (insert username to remove here)
sudo fdesetup remove -uuid (insert uuid of disk password user here)
Hope this helps!