Is this a Cucumber?

[lar]

He hitchhiked on a love rock.
About 4 to 5 inches long, Soft, dark grey in color. It has soft spikes all over it's long body like a old WWII ship mine. He stays off the substrate, Only crawls on the rock, likes to find holes and crawls in and out of them at a snails pace. It survived a hour out of water before I discovered it at the bottom of the box I transported the rock in.

 

[krazzydart]

it could be a slug....... keep an eye on it........

 

[markcolechin]

It sounds to me like a small bristle worm.....might be.
If in doubt get a good book and see if you can spot it.
But be on the safe side and dont touch it with your hands. It could be anything, remeber there are millions and millions of creatures in the sea, but you should be able to find the type of species.
Good look

 

[lar]

It is not a bristle worm, It is as big around as your thumb and can be touched, feels a little like a leather coral but softer. The spikes extending from it's body are soft as well and bend over with small pressure.