The primary problem will be maintaining contact. The maritime technology used, AFAIK, was up to a crossing in good conditions, but not adequate to ensuring regular transits. So even if you assume a successful crossing and return, the colony may not be able to get reinforcements or bring back trade goods. Of courese there is no real reason to think they would want that, either. If they are a settler colony, they might well be happy to be left alone.
What comes next depends on a few iffy factors. First, how many settlers came? A small number, even a few hundred, can simply dissolve into the native population never to be seen again in the course of a few generations. Ifrr we trust the historical accounts, we are looking at thousands or tens of thousands, but numbers in non-administrative historical records have a way of inflating beyond all recognition. Your call, I guess. Next, what is your position on the terra preta culture? I don't know enough to say one way or the other, but it will make a big difference whether your settlers meet a flourishing quasi-urban rainforest civilisation inland, or whether they run into scattered tribes augmenting their foraging activities with occasional agriculture. Of course, ten thousand Malians walking into the cities of the Amazon are much cooler than a handful of half-starved sailors joining a stone-age tribe.
Cultural contact should get interesting, though very likely tragic. Even if the Malians behave better than the Europeans (and it's hard to behave worse), eventually they'll bring across old-world diseases. The social disruption these epidemics cause could well lead to the questioning of a lot of assumptions the Native Americans had, and prepare the ground for mass conversions. If they manage to hold on to the fabric of their society (which is conceivable - many of the Northeastern agriculturalists did it), you could see a Muslim Amazonian culture with Afro-American cities lining the jungle riverbanks surrounded by thriving orchards. Mali is not at the heart of Islamic civilisation, but it has enough to offer to make the package attractive (ironworking and writing come to mind). It is also heterodox enough to allow for an easy conversion.
All of this is assuming the records are true.