In the second measure here, (skipping the first note) it's G to F# and then C to B. While this isn't the Bb A C B of the B-A-C-H motif, it's still the same sort of paradigm. He has a half-step from G to F# and then a half-step from C to B. Two sequential half-steps are also heard in other places in the work (assuming I'm hearing them correctly), although I didn't look those up in the notation, so I don't know if they're also G to F# and C to B or if they're comprised of other notes.
Among the little I know of Mendelssohn is that he promoted Bach's music and was something of a Bach revivalist, so it makes sense that Bach's musical signature would - in some form - find its way into his music, especially his organ works.
