These small clouds are a b*tch indeed...
A negative noise variation is a possibility, although you can also achieve good results with a positive value.
As usual there are a multitude of ways of doing something

Some other pointers:
1)
Contrast @ ~1
Roughness @ ~1.25
if
Contrast @ >1
then
Roughness @ < ~1.25
Small increments of 0.1 - 0.25 advised.
2)
Attach a "colour adjust shader" to the output of your cloud fractal.
Raise black colour from 0 to 0.05 or 0.1
Raise white colour with same value.
This removes tiny speckles around your fractal.
Also lowering gamma may help greatly.
3)
Lower cloud gamma coverage in cloud node