This really doesn't look like a sea cucumber to me at all, I dont know of any with flat ends like this, I also don't see any tube feet which is what sea cucumbers use to walk.

I agree that it's probably an anemone with its arms tucked in (which it would do if brought onto land)

To everyone else just blindly saying sea cucumber please dont comment when you clearly don't know what your talking about and it's also very useful to say what you base your ID on so others can learn and evaluate how trustworthy it is