That's really cool! :) It's very cute. I'd love to see more pics as it grows. What's the other little plant, do you know?

Other is a start of a columbine for outside. =)

spath as in peace lily

If it really is a Spathiphyllum seedling then that is quite something. It's hard to find info on the Seeds but some sources maintain they are sterile and others that they need a pollinator. I'm a bit dubious myself about the id. The stem just doesn't look right to me. It looks as if it it could be some sort of tuberous or bulbous plant which got into the potting mix such as an Arum or Allium. When you pulled it up did it just have roots or some sort of tuber/bulb? If you rub it does it smell of onion? Is the leaf soft textured or leathery like a spathiphyllum?

It is not a allium, they have very thin narrow headline leaves

Teengardener1888 - not all Alliums have thin leaves. Check out A ursinum, A tricoccum, A moly, A karataviense and many others. The picture has a white stem which looks very Allium like rather than Spath. I'm not saying it IS an Allium, only that it is a possibility.