Marx's demographic numbers did not look right to me, so I consulted the Wikipedia page [Demographic history of Jerusalem](https://en.wikipedia.org/wiki/Demographic_history_of_Jerusalem). Sure enough, Marx's numbers differ substantially from the numbers of the official 1851 census listed there. However, modulo a bit of round-off, they are a perfect match with the numbers given in this publication, which is also listed:

César Famin, *Histoire de la rivalité et du protectorat des églises chrétiennes en Orient.* Paris: Firmin Didot Frères 1853.

On [p. 49](https://archive.org/details/histoiredelariv00famigoog/page/49/mode/1up) Famin states that the resident population of Jerusalem comprises about 15,500 souls, of which 8000 are Jews, 4000 are Muslims, and 3490 are Christians:

> La population sédentaire de Jérusalem est d'environ 15,500 âmes:   
> Juifs: ................. 8,000   
> Musulmans ... 4,000   
> Chrétiens ....... 3,490   
>    
> Total .............. 15,490

He then proceeds to give a numerical breakdown of the Christian population. On [p. 50](https://archive.org/details/histoiredelariv00famigoog/page/49/mode/1up) he describes the Muslim population as follows:

> Les musulmans, qui forment à peu près le quart des habitants de Jérusalem, sont ici les maîtres en toute chose. Cette population, composée de Turcs, d'Arabes et de Maures est rude aux chrétiens et aux juif

[My translation] "The Muslims, who make up slightly less than a quarter of the inhabitants of Jerusalem, are here the masters of the entire thing. This population, composed of Turks, Arabs, and Moors is harsh on Christians and Jews; [...]"

I conclude that Marx simply translated, somewhat freely, these two sentences from Famin's book into English, turning French "Maures" into English "Moors". According to my 1998 dead-tree edition of the Larousse Unabridged Dictionary French / English this matches modern usage.

The French Wikipedia defines [Maures](https://fr.wikipedia.org/wiki/Maures) as follows:

> Maures [...] désigne les habitants musulmans et arabo-berbères médiévaux d'Ibérie, de Sicile, de Malte et du Maghreb, et à l'origine, durant l'Antiquité, les populations berbères d'Afrique du Nord, tout particulièrement du Maghreb.; [...]

[Google Translation] " Moors […] refers to the medieval Muslim and Arab-Berber inhabitants of Iberia, Sicily, Malta and the Maghreb, and originally, during antiquity, the Berber populations of North Africa, particularly of the Maghreb.; [...] "

Marx's "Moors" thus denotes the population group from northwest Africa mentioned in the question.