Lettuce Sea Slugs: color changers?

I've had a Lettuce Sea Slug for a couple of months now, and I find him quite entertaining. I know that they are herbavores, and also use cells from the stuff they eat to gain energy photosyntheticly, which can change their color. However, mine changes color quite frequently, as well as size.
When first bought, he was dappled tan a grey like a clam mantle. He then turned cream with blue spots along his fringes (which I liked best), then brown with cream spots. At the moment he is almost completly very pale green, with light brown spots, and seems to be flattening out and getting longer, and is much more moible than usual.
Anyone know if this sort of thing is normal? Is it in decline, or growing?

(BTW, a LSS should be kept away from Zoas with long tenticles, as they'll readily try to drag it in to the mouth. had to perform a rescue once....)
 
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