Help!! Is this a Sea Cucumber or what?!

Looks like some sort of cucumber to me. I've always had Atlantics in my tanks and they look similar. .. but don't have all those little long tentacles sticking out on the top and sides

 

Do a search on:
Eupentacta quinquesemita

 

Well its definitely some kind of echinoderm since it has tube feet.

E. quinquesemita looks to be a subtropical species, so I'd doubt it's that--though there are definite morphological similarities.
Most cukes don't come to a sharp point or have a 'collar' like this, which makes me wonder if it's a starfish that got chopped up, but I don't see a radial canal or an oral disc on the ventral side.

So basically I have no clue. Neat find.

You could get lots of pictures and send them off to Ron Shimek for an ID.

 

Looks just like the ones I have in my tanks. They came on Florida live rock. I love my corals (as we all do) & I've never noticed it messing with anything for years. I too usually see them when the lights go on.

 

Pet it and see if it's nice.

 

I've ran floridas in my tank for 12+ yrs. Half of which I've collected myself when I lived or went back to visit there. I've never seen a Florida /atlantic with tentacles like that.

 

Floridas look like this...similar...but no tentacle extension or even presence like the other one.

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Sure looks like it to me. Like they say on Liveaquaria, "Due to variations among each species, livestock may not look identical to the image provided."

 

I will add this is one reason why I love this hobby. The entire ecosystem is fun to learn and observe.