I see what you're talking about but it's too small/difficult to see in that image. Now unless my eyes are tricking me and I'm just seeing algea, I see a very blured aiptasia anemone.. or some kind of nudibranch on the rock below it. The backround info also suggest a crownless feather duster.
I don't believe it's a transparent cucumber.. I'm sure they exsist, but I'm not sure they're seen in the hobby.