Melody Credits are credits for free trial that are used to generate melodies.
In free trial version, you can get 30 melody credits. And one credit is used per melody.
When using the Auto Melody function, it creates 3 melodies at once and uses 3 credits.
You can see the balance of credits and used credits in the credit bar each time you use them, as shown in the image below.
You can check the balance of credits in the upper right of the melody section on the MUSIA plugin, as well as you can find them on the profile settings page on the website.