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 answer is hope this you.

a historian researching the immigration policies of the early 1900s would expect a source written today,rather than in 1910 to :

( a ) be more interested in objective data and statistics.