I have a mystery plant!

I was watering my green onion plant today and noticed something on the side of the container. I looked a little closer and it looked like some growth. It was this small green sprout looking thing. So, I'm not sure what it is. I was trying to think where a seed could of made it in there and really couldn't remember anything. Then I remembered that when I bought it it had been in a cluster of different plants. So it possibly could be mint or parsley. I'll have to keep an eye on it and see what happens.

Here is a picture. I'll make sure to add more later on as it starts to grow further.

Most of all I hope it's not a weed...

Update: taken the following day