OP said lives in northern California.
Funny, I just found something similar and asked about it on here too. It was growing on my patio where I've been dumping my aquarium water.
Mine is much younger though as of right now, they are much smaller than yours and to me resemble Baby Tears or Moneywort... I don't know for sure though. Here is a picture of the ones I just found and I'm in southern California.