What is right there?
It looks like a swimming bell of a #Rocket-ship_Sipho, here is an image example of another Rocket-ship to compare:
They may be missing their long branching tentacles in some life stages. The opposite end of the swimming bell may be pointy like here, or round. Look at the second photomontage in this thread to see, what the round ones usually look like.
Enjoy the classifying 😃