I've got a theory on the line: you constructed the luma matte out of video from a camera that recorded in low color resolution; a DSLR, perhaps. Chroma keying is all about the edges, and such cameras aren't great with edges.
I bet you can lose the line byprecomping the video & matte to knock out the background, then applying AE's Refine Matte effect to the precomp.
As to "schmutz" -- it's a Yiddish word that roughly translates to "icky, undesirable stuff" . You have some at the bottom of your matte, but you have a plan to remove it.
KGAN (CBS) & KFXA (Fox) Cedar Rapids, IA