Black beards when he sees me, but stops if picked up?

[Dragonache]

Hey,

My dragon, Nanotech, is a year and a half old.
Recently he's developed a new habit, if he sees me through his viv he looks at me and his beard goes black (not puffed out though) even though his temps are ok and he's very warm to touch.

But if I take him out and carry him with me, the black goes away within a minute and he enjoys being with me.

Does anyone know why? He hasn't bitten me, hissed or puffed his beard out, he just watches and goes black. :?:

 

[Dragonache]

Now he's asleep on me, he fell asleep in his sandy normal colors and has progressed to dark and black beard, with stress marks, while asleep on me. Now that he's waking up he's turning yellow again and not black bearding or showing stress marks. Does anyone have any ideas? :?