if you don't mind 'elbow grease' use dish soap and white vinegar with a stiff bristle brush preferably small bristle and rinse clean

looks like you'll have to take the whole unit down to access what ever light it uses. what happens when you apply down force to that outer ring bezel thing? hanging around elec ppl on construction jobs they call those 'buckets' that are recessed into the ceiling and they would just pull down to gain access.

yeah if you could find some kind of mesh (as per davidmlewisjr) and paint them as close to the color of the ceiling that would be ideal I'm thinking

not unless someone left a hose running in the rafters