:unsure
I found the following on WetWebMedia...hopefully it will be of some help. I know our fairy wrasse builds a cocoon around itself when sleeping (normal), but it sounds as if the cleaner wrasses are different:


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Cleaner wrasse in nocturnal "cocoon"
Bob,
thanks for answering and publishing the daily Q&A. Hundreds of us can learn
from your response to one Aquarist.
tonight I came home and inspected my 54g reef/fish tank by illuminating a
dimmed room light.
To my alarm, a basic "pacific" cleaner wrasse was on its side, a bit
twisted, at the rear bottom of the tank enclosed in a transparent sort of
slime "cocoon". The slime substance appears not unlike what I have observed
being shed from certain corals such as a scroll pagoda I keep. A Yellow
Tang was exhibiting some behavior perhaps related to the enclosed Wrasse.
He was hovering and backing into the "cocoon".
Do you have any idea what I am observing? Does the wrasse exude some
protective slime in which to sleep?
Could some other predator critter excrete the slime to paralyze and later
consume lethargic sleeping fish?
I should tell you that I have lost several fish to "mysterious causes" whose
carcasses are never found.
I devour so much aquarist material - both on line and hard copy - but have
never run into a description of what I am observing tonight in my tank!
Perplexed, Richard Buonomo
>>
Interesting... don't know that I've ever heard of a Cleaner Wrasse (likely a Labroides) exuding a sleeping cocoon, though many wrasses and Parrotfishes do... And there are a bunch of organisms that might use such feeding techniques that might render your observation... And it does sound like there is a "mystery" culprit in your tank... but who? A sea cucumber? Large polychaete worm?
Bob Fenner
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It may be of some help to find out the exact technical name for your wrasse and see if this is normal behavior. I know you mentioned he's "just about gone", but is this observation just due to the cocoon, or does the fish itself look bad also?

W.