These are seedlings, most likely from the Euphorbia in your post "weed photos". I'm not sure what types of Euphorbia you have in Riverside, if it were in my garden I'd say E. polychroma. I won't reply to the other post so it will stay on top until somebody recognizes your specific plant.

Also, you might want to wear protective gloves when you weed this plant, the white sap can be an irritant to some people, especially those with latex allergies.

I agree on Euphorbia - in my garden it would be E peplus, Petty Spurge, an annual, unlike P polychroma.