I generally find lifeforms that count on magic creatures like golems or the like common enough that things evolve to prey on them rather unlikely. A better explanation is this a weapon built to destroy such creatures, which opens up the Who and the why questions.
So accepting this is a magic weapon to destroy golems, having a side effect of conversion into monsters is somewhat more plausible, but simply leaving a patch of green where once stood a golem, or a half-consumed golem lashing out blindly are less odd to me :)
Fog can have different aspects. Ditch the elemental purity and it can be corrosive (acid fog, or at the very least rusting), it can be poisonous or even deadly - perhaps the fog opens a connection to elsewhere, allowing things from beyond to enter, but only the fog.
Fog could be a lifesaver in the desert - it blocks some of the suns rays and cools. Deadly on the cold north seas where it freezes to every rope, sail and board.