I'm not very great at triggers but how about making it so that each time a player gets near the location where the text is supposed to appear, make it appear. Then, whenever there is no respective unit nearby, the text disappears.
Well... I have no idea how to make the fog more visible. XDI think a cool little effect would be to make the fog more visible the closer you got to it, so you set the point of the text, then you make a trigger that's like, a unit within range of (the floating text), set transparency of floating text to (distance between triggering unit / 2)
Or something like that, it's been so long since I've use WE I've forgotten how to explain it, but if you're good enough with WE i'm sure you can figure it out?