Powdery mildew [the white powdery]. Also possibly downy mildew [the yellow splotches].

Aw maaaan...DM AND PM? :(

So wait...if that's what DM looks like....is this it as well? (cuke leaf)

Turn it over. Look for a gray-brown splotch opposite the yellow spots.

Getting both PM and DM isn't all that common, because they thrive under different conditions. But cucurbits are mostly huge disease wells, and all gardeners can usually hope for is to get some crop before they totally succumb.

I would take immediate action to spray with anti fungus spray.
PM is so obvious. if there is also DM, spray should take care of that too.

PRECAUTION:

I would also spray the cukes , just as preventive measure. I do not know about DM, but PM is air borne and thus can infect other cucurbits around.