That is its aft end.
It seems it has left its tube. This may be an indication that it is sick or that it doesn't like the place you put it. They prefer slow but steady flow, low light and deep sand.
Should it not get back in by itself you may need to provide it with an artificial tube. Appropriately sized PVC tube ends work well for that. Just make sure to deburr both sides of the tube well, so that there are no sharp edges that could injure the anemone.
It will rebuild its tube, using the PVC end as start, if it is healthy and likes the place you put it.