This nasty looking spot occurs on a very limited number of hybrids that have tetragonum variety tetragonum in their background.
It is a genetic fault and cant be cured.
It doesn't seem to effect the orchids health, just its appearance.
Older Hilda Poxon and Ellen seem the most prone with 50% tetragonum.
Sometimes the tiny spots are invisible in the background of the leaves,but come out heavily when the clorophylle in the leaves is weakened by frost or hot sun.