What does the small diagonal line between the two notes mean?
THe OP leave us guessing where this came from, but assuming it is Chopin's ballade in G minor Op 23, it is simply a "chord" consisting of B flat and B natural.
The 19th-century notation for this was just two notes on the same stem, with two accidentals (flat and natural) in front of them. In the 20th century some composers started using two separate notes joined by a "forked" stem. The OP's graphic looks like a poor attempt to reproduce this with an (unknown) notation software application that can't do it right - or the user didn't know any better.
Breitkopf und Härtel (1878) has staccato dots over the notes, which somebody may have mistaken for the slanting line:
More modern notations:
Looks like a slide. You can't do that on a piano, but you can emulate smearing from B natural to B flat (which I presume this notation is supposed to suggest) by striking both right after another and releasing the B natural after striking the B flat and then release the B flat "regularly" on time.
Probably intended to render a "blue note".