Are you asking about the US or all of the Americas?

It is impossible to give a percentage. Muslims were documented in North and South Carolina (Wood, Black Majority). Muslims probably were in Brazil but resisted baptism therefore they would be absent from Church records and then are almost impossible to find and count today (Hall, Slavery and African Ethnicities in the Americas).

Also the time period would cause variations - are we talking about 1550 or 1750 - because in the same geographic region of Africa changed religions in this rough period (Thornton, Africa and Africans in the Making of the Atlantic World).

Most/all African slaves were from Western Africa. Islam was/is predominantly Northern Africa, so while it's not impossible for any of the peoples in Western Africa to be of the Islamic faith, it's highly unlikely.

Also, in the United States at least, slaves were forbidden from practicing anything other than Christianity. This doesn't mean that other religions weren't practiced or mixed (voodoo, etc).

Sources: Making of the West, Hunt et al, Slave Religion: Raboteau.