Since it doesn’t look like there is an answer for why it’s so bright, I thought I’d try to explain what is happening.
The reason that it is so bright is that colors are allowed to be whiter than white(1,1,1) blender. And the object coordinates are based on the size of your object, so if your mountain is 10m then the color will be (10,10,10) at the top. Effectively reflecting 1000% of the light that hits it, I’m assuming that white reflects 100% of the light that hits it and it scales linearly.
Oh, I almost forgot. The reason that it’s glowing is because of EEVEE’s bloom setting making pixels brighter than a threshold glow, mixed with the fact that everything is basically over exposed.