I would need to see more pics but it appears to be a weed. Common in potting soil, especially from miracle grow or promix. What happens in those bags of soil is wild. There are crazy amounts of bugs in those bags and you get the stray seed or 2 from weeds that don't get burned out from the time release nutrients.
If you can take more pics I can tell you for sure. But from what I see it's the same look of the weeds I get in my container plants. The only other option I see is maybe a plant that's called the "ice plant" sold in many greenhouses. Let it grow and see see if it flowers. Once it flowers you'll know for sure what it is by searching for the flower on the net or posting a pic on here.