Granada was conquered by Ferdinand and Isabella not long before Columbus. I suspect the only reason the Catholic Monarchs gave Columbus the funding for his expedition to the Far East (when every other ruler of the time rejected him due to the world not being as small as he said it was) was because they were too engrossed in celebrating their victory to notice that detail.

Other than Granada, most Islamic powers with significant navies (such as the Ottomans) were located too far away from the New World to have been able to make an effort. Maybe the best shot of Muslims discovering America would be if there was a war against Japan in the Yongle period, and Zheng He was tasked to lead the naval operations but got blown off course to Alaska or Canada.

I suspect Muslims would have been as horrified at Aztec human sacrifices and other New World religious elements as the Catholic Spanish were, and which might similarly cause the Muslim conquerors to consider the natives to be barbarians. However, the Muslims might have a different impression of the desert-dwelling Indians to the north.

I think smallpox and other foreign diseases would make just as bad of an impact on natives in a Muslim-ruled New World as it did historically.