I don't know what this substance is but in the 90s there were lots of electronic gear that was encased with this rubber coating because it gave the device a "tough" look. Years later we know now that stuff is no good; it decomposes over time and turns to greasy sticky tar. Elation techs say the company doesn't supply lights coated with this substance anymore; they use a different plastic panel supplier for their products now, and they were not willing to say what the substance was or offer any advice how to clean it off your lights - basically if you own a lot of these light fixtures from 12- 15 years ago that have deteriorated covers you are on your own.
Take off one panel and soak it in hot water and Dawn as a test. May take up to 48 hrs. If it works and the glop comes off, do the rest. The white splotches you see is leftover soap that has not been rinsed off yet. You need vigorous rinsing with water and a good wipedown with a fresh rag; the cover will come clean. I did 4 lights this way.