What was a negative side to having the rest of the world (Meaning Europe) learn of Mali through the travels of Mansa Musa?

Probably because when more people learned of Islam and were influenced by Musa (RA), less people would be Christian and become converts to Islam, most likely because they would associate his immense amount of wealth with being Muslim, and seeing how most of Europe back then were Christian and opposed Islam, they'd probably see this as a blow to them. They could probably associate his vast empire as being the effect of converting to Islam as well.

"the attempts of the proletariat to rise above this abasement"