1870 Morgan Twp, Coles Co, IL. At census time he was living with George J Collins, Lucinda's uncle.
1880 Nevins Twp, Vigo Co, IN. That census record is very faint and hard to read but it looks like Alexander and Lucinda were both 35 while daughter Martha E was 12. Alex was a laborer. In the same neighborhood was Abraham Harp, possibly Alexander's father.
There is a Harp grave next to Sarah Collins in Union Cem, Coles Co, IL but needs a rubbing to read.