It may not be the password itself. If you’re seeing “ssh@USERNAME.mph” then I believe USERNAME is where your USERNAME should be getting placed, unless you’re just saying USERNAME to block out mentioning your username, which I understand if so. You can see what I mean from the “Log in from another computer” section of this page.
Could you ensure that the Remote Build Options are being set correctly before you hit Generate SSH Key? RSync User Name should match your Mac’s username. If -mph (as you mentioned) or -mbp (as shown in your picture) are not part of your username or server name, they shouldn’t be showing up either.
The password should match your login password for your Mac, so resetting that password would be the only way to reset it.