My guess is that its dried skin and that as its been warmer, their skin may be drier than normal. Perhaps added to that they may not have been bothering to groom so much.
After all, when its hot all you want do do is enjoy the sun and lie in it, or if you are too hot you sleep in the shade. Not exactly major grooming session weather
Both Mr T and Chloe are showing some "kitty-druff" in the harder to groom areas.
