The problem here (aside from the fact that your screenshot is tiny) is that slope sectors can't be cut in real time by FOFs. The entire slope floor must be drawn at once, which prevents it being cut.
I'm tired right now and can't give detailed instruction, but the best way to fix this is to split the slope sector over the line you want the slime to come over the top of it, then copy the slope from one side to another. Maybe someone can gove you a more detailed explanation before I properly wake up, or maybe you get what I'm getting at - but if not, I'll try explain things further later.