Pink Cucumber not doing much

I bought a pink cucumber from the lfs about 2 and a half weeks ago, when I placed him in the tank he sank onto the sandbed and the over the next few hours climbed to the top of the tank on the front glass. He has been there for the whole time.
He frequently has his feelers out and retracts them if there is any movement nearby.
I noticed that he was starting to get smaller, and narrower in the mid section of his body, so I have been letting the Live photoplankton that I feed the tank drip slowly from above into the water. Since I have been doing this he has started to get slightly bigger.
Is there anything else I should be doing for him, as he hasn't moved at all in 2 weeks, apart from placing his body vertically the other day. Also I have heard some horror stories about these nukeing tanks.

 

That's pretty much what filter feeding cucumbers do. They find a place in your tank where they get the kind of flow they like......then they stay there and eat.