Ishmael and other characters repeated refer to Queequeg as a cannibal, which in modern parlance means that he consumes human flesh. However, the novel doesn't ever say that outright, rather, Ishmael just seems to use the word to indicate that Queequeg was just non-native from a distant land that practices a bizarre religion.
Is there any indication that Queequeg was a cannibal in the modern sense?
Queequeg was indeed a consumer of human flesh. The relevant passage comes from Chapter 17, which describes Queequeg's religious fasting.
I then asked Queequeg whether he himself was ever troubled with dyspepsia; expressing the idea very plainly, so that he could take it in. He said no; only upon one memorable occasion. It was after a great feast given by his father the king, on the gaining of a great battle wherein fifty of the enemy had been killed by about two o’clock in the afternoon, and all cooked and eaten that very evening.
“No more, Queequeg,” said I, shuddering; “that will do;” for I knew the inferences without his further hinting them. I had seen a sailor who had visited that very island, and he told me that it was the custom, when a great battle had been gained there, to barbecue all the slain in the yard or garden of the victor; and then, one by one, they were placed in great wooden trenchers, and garnished round like a pilau, with breadfruit and cocoanuts; and with some parsley in their mouths, were sent round with the victor’s compliments to all his friends, just as though these presents were so many Christmas turkeys.
Whether or not it was common in 19th century to indiscriminately refer to all foreign tribespeople as "cannibals", Melville explicitly characterizes Queequeg as a genuine cannibal.
