Translucent hitchhiker

[spaceman spiff]

I just recently set up my tank and added some LR from the LFS. It had been in a display tank they had set up for quite a while, so I wasn't really expecting much in the way of hitchhikers. I got it mainly to seed the rest of my rock.
Anyway, most of the hitchhikers I have noticed I can easily identify. Like the brittle star and the snail I saw tonight. But I've got one I can't.
All I can see is his arm/tentacles. They're translucent and they sticking out of a hole in the middle of the rock. They're poking around looking for food. Can't see much of him because of the way the rock is positioned. I know it doesn't like the light because he only came out after I turned them off. He ducked back into his hidey-hole the second I turned them back on to get a better look.
I'm not too concerned about him, but would like to know where to start looking to try and figure out what he is. Any thoughts?

 

[tizzo]

if you see it in the light it may be an aiptasia without any zooxanthellae
edit: One day I will learn to read. There is a shroom that is nocturnal called carriboreum (sp?) that is clear with little white balls on the end of it's tenticles.
Also one called a hidden cup coral.
They look alike except the hc has a skeleton.
I'll look for the correct spelling of that shroom

 

[tizzo]

pseudocorynactis caribbeorum