As an organist-amateur I recently bought "An Easy Bach Organ Album". In BWV 691 there's a symbol which looks like a half of a circle, or a small letter c. Moreover, the symbol is connected with the next note. How is the symbol called and what does it mean?
The "c" is a "slide", which would be played as F# G# A (starting on the beat). That would then be followed by the mordent, so the whole ornament would be F# G# A G# A played (approximately) as four 32nd-notes followed by a 16th.
But that sounds pretty crude musically, because the collision of the F# on the beat in the bass and treble.
The "slur" is probably intended to prolong the note before the ornament, which removes the weakness and would be played something like this. Of course ornaments are not meant to be played in strict time, and this implies a tempo slow enough to let all the notes "speak" properly, and let the listener hear the F# bass on the beat against the G# in the treble before the ornament starts.
The Bach-Gesellschaft Ausgabe edition has the "slur" but not the "slide", and does not have the G# grace note in Les Éditions Outremontaises.
There is a more longer and more elaborate version of the chorale prelude, BWV 691a, which has no ornament at all in the Bach-Gesellschaft Ausgabe edition.
So (as with all baroque ornaments) at the end of the day you play whatever version you prefer - and not necessarily the same version every time you play the piece.