Granulation is a characteristic of some pigments which occur when the pigment particles settle into the paper creating a mottled effect. Several artists actually desire this quality because of the visual texture it provides.
Granulating colors will be marked on the tube with a G.
Flocculation - to form into cloud like masses. tufts of wool.
Granulation Medium provides a granular appearance to any colors desired. If you add this medium to already granulating pigments, the effect will be enhanced further. Rough paper vs. hot press will also help enhance the effect.
For maximum effect, dilute pigment in medium alone. dilute pigment further with water for more variety in the effect. I find it helps to shake or tap the painting a bit to encourage the medium to work it's magic. Always keeping the watercolor horizontal.