Donkey Tail Spurge
Euphorbia myrsinites in bloom. You have to get up real close to see these blooms. Often the bracts on the Euphorbias are quite large and attractive and so are mistaken for the blooms. But if you get out your microscope, or maybe just a magnifying glass, or even maybe just put on your reading glasses, you can see these lovely little things.
In the case of this particular photo, you can also just click to embiggen.