This has got to be the most weird thing I have seen in my tank.
I have seen it both in night and in day, but it seems to look the same - almost like an anemone stalk, it is gelatinous with some white streaks here and there (it does elongate slightly at times). The only difference is that it shoots out these thin spaghetti tentacles when it is pissed off. I retracts them into its 'mouth' when it is relaxed, so it is not totally expelling stuff like a sea cucumber.
I have pissed it off a couple of times, once when I moved its rock to get a better look at it, and then today I tried to feed it with a syringe, but when it felt the water current full of dead things hit it, it shortened up a little and spewed the tentacles at me.
If it is that important, I could probably capture that on video, although I hate to piss it off again.