It's leafminer damage, and pretty serious to boot. I suspect you are dealing with Spinach leafminers which, in spite of the name, love feeding on chard/beet/etc.
Those opaque areas you see are the "mines" where the larvae feed: it's dead tissue that will later turn dry. Use a spinosad based insecticide since it's effective but has minimal/no toxicity on bees, puppies and leprechauns.
