Civil War era derogatory slang for a black person. This was probably derived from 'mocha', i.e. the coffee named after the port in Yemen.
By the way, we visited Captain Armes' Battlefield up there toward the Republican (River), and found one dead horse and the remains of one dead negro! It appears that Armes
didn't find the Indians after all, so much as they found him! .... he managed to get out of the scrape after a fashion and the whole force drifted back to Fort Hayes without any procrastination .... The Kansas troops are a set to contemplate with curiosity--and the 'Moaks' are in the same category!" (Life in Custer's Cavalry, ed. by Robert Utley, Yale University Press, 1977, p 99)