Thanks, everyone, for your replies. I found out, thanks to Petalpusher’s mint suggestion, that it is called Clearweed or Richweed. (Pilea pumila) Google is wonderful, isn’t it?
I was pretty sure it wasn’t a mint (no smell, short roots/no runners) but that gave me a place to start looking. One website mentioned that nettles have the same square stems, opposite leaves, etc. and are often mistaken for mints. Soooo, went looking through the nettles websites and there it was! Clearweed!
Pulling is probably the only way to control it now. This area is about 40x60’ and has over 200 hostas planted in it. Can’t dig them up to spray. And Primroselane, believe me, if you don’t have these weeds, be careful not to GET them! They are soooo invasive!!

I plan to recruit my boys to help pull all that we can (AGAIN) before it blooms! Oh, my aching knees…. I have gotta get some mulch!
Thanks again for your help!
gljo