Use of IS and WAS in reported speech [duplicate]

Both are correct.

Melissa said she WAS going to the mall.

This implies that at some point she had informed you that she was going to the mall. Whether she went there is ambiguous.

Melissa said she IS going to the mall.

This implies that she is on her way to the mall.

Depending on the situation, either could be right.

For example, you're in college and you're sitting in front of the TV and it's late and your roommate asks where Melissa is. Your response would most like be: "Melissa said she was going to the mall.

If you're sitting in front of the TV and Melissa has just stepped out the door after telling you where she was going and your roommate asks where she's heading off to, then your response would likely be: "Melissa said she is going to the mall.