Allison, I wish I knew. So many song books make statements like that above the music, but don't give the source composition. Frustrating!

Sometime ago there was a thread on a song, and the reference was to Bach. It was finally traced to a secular cantata, a tuneful one but seldom heard.

Looking through the Bristol Tune Book, there are many similar examples.
Drayton 8.8.6.8.8.6 Moravian (one presumes a hymn or folk tune)
Lubeck: German Chorale
Nottingham: From Mozart
Redhead No. 47: Redhead
Now if the Opus number of the Mendelssohn piece had been added, it would be easy to look up.