Can you please tell me how melanoma and malignant melanoma differ?

No difference: These are equivalent terms because there is no such thing as a benign melanoma!

Same thing: ...Just one is shorter to say. There's no such thing as a "benign melanoma", for example.

The same: Melanoma is be a malignant tumor. Sometimes people only refer to it as melanoma and other times as malignant melanoma. They are the same entity.

Depth: Generally these terms are used interchangeably, but there is one exception. This is for melanoma in-situ which is very early stage and termed pre-invasive. If neglected melanoma in situ will grow and become malignant.