The patch is a boring sawtooth, missing some low harmonics like a clavinet. However it runs through a triple flanger whose frequencies are controlled by LFOs, that in turn are controlled by the pitch bend. The flangers fade out when they reach one extreme of their time range, and fade in at the other end. This gives the unusual effect of a flanger going up or down forever. Note that this triple flanger isn't built into Moselle. Not even a flanger is built into Moselle. It's just the kind of thing you can make if you get the idea for it.
Note the sound is recorded directly from Moselle: no hardware or external processing was used.