the white part is where the haze appears. To get this part really looking good, you have to increase the haze and the hazealtitude. Another thing is, if the horizon is visibel but there is nothing. Then, just duplicate your terrain, rotate it and use it to fill the background. Or put an infinte plane and make it to water. Here in this image the horizon is nearly as high as the top of the mountain on the right. So you will need a lot of fill up. This is no bug. It is a simulation of the world and so, if there is nothing, nothing (white nothing) is to see.