I have found it can take two winters to germinate some clematis seeds though don't know about c. mandshunica. Perhaps the pictured plant is a weed that blew into your container. I would take it out of the pot and grow it on until it was recognizable or determined that it is a plant I want to keep.

That does look suspiciously like mint. Have you scratched and sniffed? What mix did you use? Was it recycled?