I think it is just that no one knows. You stumped Bang Guy! That should tell you something...heh heh
I do think it is a type of flatworm, but it doesn't resemble any that I have seen. There are so many different kinds of flatworms, it is hard to pinpoint.
on the topside does it have two eyespots???? looks very similar to planaria (tha actual planaria, not the flatworms that are generalized as planaria!)!
good luck iding it!
jon