Land it as normal. That is what I would do if it was attempting to land “on” the fog then continue the flight as normal.
The Spark will not descend thru fog...it's impossible to land, as far as I know. I got caught above some fog once, and had to fly away a couple hundred yards to where the fog was thinner, and descend down over there.
I suppose it's possible that when the battery gets low enough, the Spark WILL descend thru the fog.