This isn’t an insect but is probably a disease. Spinach is prone to downy mildew and it’s the most common disease on spinach. Although it usually shows the white growth on the underside of leaves, it’s sometimes also seen on top of the plants as your photo shows. Without culturing a disease situation in a lab it’s impossible to be 100% sure, of course, but knowing that downy mildew is a common problem on spinach in California we feel secure that this is most likely what you have.
Unfortunately there is no cure. If this is indeed downy mildew the leaves will also develop yellow spots or become distorted and rather sick looking. Sometimes the underside of the leaves turns dirty looking. You might want to grow a different type of green for awhile and let the downy mildew spores die out. Plant spinach further apart in the future to allow for good air circulation. You can read more about this problem here: http://www.ipm.ucdavis.edu/PMG/r732100111.html
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