Your picture isn't really close enough for me to be sure and I don't know where you are located, but I suspect your plant is Lippia citriodora aka Aloysia triphylla aka lemon verbena.* Is this it?
I have it in my garden and it will seed around the garden if you let it set seeds.
When weeding, the best way to make sure you are removing a weed and not a valuable plant is to pull on it. If it comes out of the ground easily, it is a valuable plant.