... L.Q. "Sonny" Clemonds points at Data and asks Riker, "What is that?" Then Data turns around to see what he's pointing at.
Data must look particularly artificial in person, more so than on the TV screen. There must be something about him that screams "artificial" to an individual from a more technologically advanced society. When he was stuck in the 19th century, no one seemed to think Data was not a human, just that he was particularly pale. Clemonds, though, immediately picks up on Data being something other than human. I wonder if 24th century individuals would initially have an even stronger reaction to Data than Clemonds did.
I think the world was considered a bigger place in the 19th-Century and that many thought that it was still full of mysterious things and people they had yet to encounter. As the term "android" (and even "extraterrestrial") was still unknown to many, the only thing Data could be taken for by most was some kind of strange foreigner, IMO.
By the 24th-Century, androids were fairly well-known (with many more human-like in appearance than Data), but all were regarded as merely machines not people. Data was an extreme rarity.