Hello Qim
"Specks" means "small marks or foreign bodies on a surface". "Fluff" on the other hand refers to a dense, minutely fibrous, irregular, three-dimensional structure, usually of organic origin. So "specks" may consist of small pieces of "fluff".
When snow sticks to a window pane, and doesn't melt, you might well describe it as "specks of fluff". Some readers might find the phrase less appropriate in other contexts, e.g. falling snow; but I'm not sure it would ever be reprehensible.
All the best,
MrP