are you serious? your on a marijuana growing website and you say it looks like a fern? have you ever seen a seedling? he planted weed seeds, so they are weed. Even if there were other things in the soil, they would most likely be small small plants like grass. So yes it is cannabis.
I had a fern sprout on the outer edges of one of my 5 gallons, right after I transplanted with brand new soil, from a sealed bag of Eko. it's probably because the Eko soil has compost in it.
ferns reproduce via SPORES, not seed, so they can be anywhere!
I took the fern out, since I didn't want it making more spores to compete for the rhizosphere. I put it with my other houseplants and it did well for about a week, and then I didn't watch it enough and full sun hit it for 8 hours and it died
cute little ancient plant
had cotyledons and everything, then got to 4 little spirally-radiating stems of individual leaflets
I wish I knew the species so I could recultivate
it was so darling