Hypothesis: since Jewish Encyclopedia states that his grandson Harav Hagaon Simcha Cohen Rapa is a younger cousin of Avraham Menahem HaCohen Rapaport (https://www.jewishencyclopedia.com/articles/12569-rapa-portrapa-sim...), one possible connection between the two families would be that this Moses Cohen-Rapa is the son of Rabbi Yechiel HaKohen Rapa, of Venice That would make R' Simcha and R' Avraham Menachim first cousins, once removed.
