Any ideas what this plant is and how to control it?
This looks like a variety of the attractive but invasive ground-cover Deadnettle, (Lamium maculatum), - see photo RHS - Lamium maculatum and here.
Deadnettle can be difficult to control. Roundup is fairly effective, but you will need to persevere - and also do a bit of hand-weeding - as the plant is tenacious and tends to reappear.