This tentacle looking thing is the proboscis of a Spoon Worm. It is gathering sand. When it has a load of sand, it will be retracted by the Spoon Worm. The Worm is in a hole in the back of a cave, and it will eat the organic matter that is with the sand.
I think that this Spoon Worm is a Bonellia minor. I cannot find a lot of information on them, but I have seen their feeding proboscis extended from the back of caves before. Amami 2026
The tentacle of the Spoon Worm leading to its main body somewhere in the back of this cave. Amami 2026