Looks like the common spurge to me and I'd be pulling it out.

Looks like a young Iily to me .Check if there is bulb under it

Johnjoe I did wonder about that but it seems to be appearing in all of Diane's pots which made me decide against...

Quick Dianeolin, pull those out! : o )

I have grown Liatris,this year,and the foliage looks similar...

Looks lily-ish to me too.
Is it a weed? A weed is ANY plant growing where you don't want it to grow.