My attic has this white fluff insulation laying on the ceiling. I noticed in my guest bathroom that when the A/C was running there was almost no air coming out of the vent in there, but there is an accumulation of white dust in that bathroom that has the same consistency as the insulation.

http://i.imgur.com/gNrhIVV.jpg

Does this mean there is a leak in the duct leading to that bathroom? Is this fluffy insulation dangerous to breath in? What is it called?

My guess would be that the duct has become detached or was never attached to the vent. Go up and look.

The insulation is relatively harmless, but likely has a ton of dust imbedded in it. You don't want to be snorting it, but you don't need a respirator to go up to the attic and look. If you decide to add more spray in insulation, then a mask would be good.