This is a large anemone, usually the host to clown fish. When my maroon clown fish started living in an atlantic anemone, I decided to try to give him a better home. He has ignored this anemone, much to my annoyance. Here is the anemone, with its foot stuck to the glass. A kenya tree coral can be seen to the left; the hippo/regal tang is on the lower right; the flower pot coral is hidden behind the anemone, but its polyps stick out over the top of the anemone; and the green button coral polyps are visible in the shadow of the anemone, to the right.
The anemone has now decided to sit on top of a pile of rocks; the yellow fiji polyps are in the left foreground.
Now, the anemone is sitting near the top of the tank, on the glass. You can see its reflection in the surface of the water: parts of it are actually exposed to air.
Another shot of the anemone near the surface. This is just after feeding the tank with freshly hatched brine shrimp, and you can see why the anemone chooses to sit here: it can skim all of the extra food which rests on the surface of the water, as clearly shown in the picture.