Another name for this pretty, delicate-appearing green monster is Garden Enemy #1. It's pilea, an enthusiastic weed which grows to some 12-18" if permitted, and which likes moist, shady conditions. Precisely what my back yard gardens offer. I'm not sure when it first arrived, probably 3 or 4 years ago, but I recall the Cooperative Extension office identified it for me.
Little did I know it would take over. Deceptively easy to pull out, with weak, water-filled stems, I nonetheless must have let a few of its antecedents go to seed. So last year it greeted the spring with vigor. And this year it's hiding under the leaves of the big hosta, it's disguised among the astilbe, and it's insinuated itself in the shady rock wall by the hydrangea. It's everywhere! I keep pulling and pulling, but alas...it's found a home.
Here's hoping it goes away as fast as it arrived. But I'm beginning to despair.