What did Mansa Musa think was true?

The question is missing a critical narrowing component. Mansa Musa necessarily thought a lot of things were true, such as the fact that gold is valuable, that salt preserves food and adds taste, that the stories of the griots were amusing and interesting, etc. Unless you want a list of everything that Mansa Musa thought, you may wish to narrow the question. If you wish to ask what religion he believed in, he was Sunni Muslim, most likely of the Maliki School.