RBTA shrinking HELP

[Surfacer]

I have had a RBTA in my 90g mixed reef for a week now and it has been doing very well up until a few hours ago. I bought it on a rock because it had found a deep hole which it refused to come out of. I got it home, acclimated it, and put it high up under my 6 T5's. I have fed it and gave it a fair amount of flow so it seemed happy. It has never moved and it is almost always out even with the lights off (though it does shrink up a little when they are off). Tonight, after the lights went out, I looked for him and thought he had left his rock and gone in search of another place. Sadly he had shrunk inside of his tiny hole in his rock. I just want to know if this is a normal thing for them to do? I turned on the dawn/dusk lights and could see his tentacls had also gotten way smaller but he seemed to react to the light slightly by perking up a little but for the most part stayed the same.
My tank is a little over a year old with pristine conditions (I tested the water and then got it retested at the LFS). I have done a lot of research and have kept a watchful eye on him but as soon as I turn my back he does this to me.

 

[Sugar Magnolia]

It's normal for them to shrink up from time to time. What kind of lighting was the anemone under where you got it?

 

[papania]

They shrink up when they expel waste, don't be alarmed. I am sure it is doing just fine.

 

[Surfacer]

It was under halides on the bottom of the tank which is why I put him near the top of my tank. I woke up this morning to find it has come out of its hole and is bubbly again which is good. Thanks for your quick replys sugar magnolia and papania. Help from those who are more experienced than I is always appreciated

 

[SPARTAN VI]

Absolutely normal for them to shrink to expel waste and when stressed. A few months ago my chiller failed and in the middle of a California summer, this meant 100F was common. However, the chiller would still run and make noises a working chiller normally makes, but I didn't notice it had failed until how many days later when I checked the thermometer.

It was flippin 89F!!

Not before long my 3 RBTAs bleached and shrunk to 1/3 of their normal size and started running all over the tank in a panic. At one point, a couple of them looked like goners. Quickly had my chiller repaired and hooked it back up. Several months later and you'd never think they were on the verge of dying.