Did you let any lettuce plants go to seed in that area this year or last year? Collard leaves generally look more similar to the leaves of other cole crops although they are more elongated. These look a little too much like the spring green color of lettuce seedlings than that darker grayish-green of collards-----at least they do on my computer screen.

Could this be young chard?

Komaosuna? Also let it go to seed. Komatsuna

The picture looks kind of like chard, Bon, but it doesn't in person. I planted a lot of asian greens last spring, but I thought the collard and komatsuna were the only ones that survived to seed. A few lettuces went to seed, Red Romaine, Chadwicks Rodan (sword shaped leaves.) This is the bed where the collard seeded. Part of it is the sun on them makes them look lighter. I can't imagine any thing else in that bed that would have gone crazy like this. The back left corner by the white pole, that plant is coming up from a root. It could be collard or broccoli in that spot. And it is all definitely a different shade of green.

Yes, the color of that plant in the back corner is the typical color of a brassica seedling and it the color I'd expect to see with some brassicas like cabbage, broccoli, collards or cauliflowers. Your seedlings might be komatsuna, which often does have a lighter green color. I don't grow it so don't know what it looks like when young. Have you tried tasting a leaf to see if it is a flavor that is familiar to you?

Yeah, I thought about tasting it, but my taste buds just say green leaf, unless it's bitter. I have this vague memory of sprinkling seeds in that bed in very early spring, when I had the itch, but I think it was lettuce. And the lettuce went to seed there, too, but it had red in it. The bed has to be cleaned out anyway, there are sweet potatoes and actual verified lettuce seedlings in there, along with bush zuchinni. If nothing else, DH works with a guy with a pet turtle that likes to get garden scraps.

Looks like mustard to me, so I vote the komatsuna.

It sort of looks a little like a Mustard. I have read that all Brassicas can cross pollinate with one another, as can all members of the lettuce family. Lettuce and Brassicas will not cross. So you may have a hybrid of some kind. If you can't eat it raw, try boiling some in water and put a little sugar and bacon grease in it and douse it with a little pepper sauce and see how it taste.