Okay, I just looked it up, and I realize my mistake: I've lived on O'ahu for years, where we have no active volcanoes. "Vog," apparently, "volcanic fog," or a form of smog formed from volcanic vapors. I guess I should have known that after all the troubles on the Big Island this year.
Once again, it would be nice if Duo offered a translation for a nine English word. Vog isn't a thing unless you are local to Hawai'i itself (see comments from other islanders above). Both of the words polalauahi and vog are foreign to English speakers and require some b translation outside of these unmoderated comment sections.