I would have said it looked like chickweed to me, but I would want to see flowers.
Corn salad, and miner's lettuce, also look similar, and are edible.
I would check with a
local person to find out if there are any dangerous local look-alikes to be concerned about. If you don't know anybody who you can have a casual conversation with, I suppose you could call poison control (the number automatically forwards to a local university extension service) and describe the plant. Maybe saying "I found it mixed in with the salad we already ate" rather than 'Can you guarantee me that it is edible'... and they
should be able to tell you if it resembles any local toxic plants.
I've got the poison control number on my website under 'backyard salad' along with some pictures of chickweed in bloom from Portland:
http://www.ernieanderica.info/gardenandedibleplants
The variety we get up here in Tonasket seems darker, and not as tender, but still tasty.
With any wild edible, you want to start with small tastes, rather than a big salad, especially if you're not sure.
-Erica W