Ahh, it does look like a mask problem.
On the chance that it is, check to make sure there are only three colors on your map's Mask:
Those being Red, Green, and Blue.
These colors, as you know, represent a certain type of terrain in your map. However, if you 'mix' colors, such as the Blue/Green mixture, Cyan, or the Red/Green mixture of Yellow, then the game will automatically render the Cyan areas as a white-ish mixture of the two Blue and Green terrains.
Any colors on your mask other than Red, Green, and Blue (also referred to as RGB) will result in the white-ish terrain you see there in your screenshot. So, you can really only have 3 main terrains in your map, even though it says there are 6 slots for it. This is a rendering thing we kind of have to live with. xD
It may also do this if your Mask terrain is too complex.
If you have a neon spectacle of RGB splee-ah all over your Mask, sometimes FH just derps and decides not to render it. If it's not the Cyan/Yellow/mixture colors I stated above causing the white terrain, try simplifying your Mask in your image editor again and see if it helps.
Good luck!
