i have heard of plecos "sucking off the slime coat" of other fish, but never actually have seen it. If it was really killing the fish, wouldn't the other fish try and get away from the pleco?
Quick update: pleco has been removed, there were a few questionable spots on tsn lower belly that i couldnt see till he got active. not taking a chance on my Thanksgiving being hurt by my pleco (even if he does like it)! lol, especially where we have space in another tank.