It's a sea cucumber. Look at its attatching feet, they go all around its body Nudis have a snail/slug like foot and often tassels, this little guy is useing what should be tassels as atttching devices like an urchin or sea cucumber.
Dean, my reply may help YOUR DECISION...a bit, I hope!
If the relative amount of dendritic tentacles (external branch-growths) are many..then most likely a PRIMARY PLANKTIVORE!
It looks like you have one here...
....You said it was with your ROCK
Decide if system maturity,and biodiversity/refugia yields are enough to healthily sustain the animal's plankton food source.
~ Will it outstrip reproductive rate of refugia plankton species, hence starve, and DIE?
~ Reduced plankton is UNDESIRABLE, BUT WORSE is DEATH OF ANY HOLOTHURID(sea cucumber) in our relatively small environments... many toxic complications from poison emission, or stress-induced evisceration( turning insides out, yeeuucchh!!!)
Also, if it proves to be a PRIMARY DETRITIVORE...which this shows few anatomical signs of, are you intending to keep it intank, where you DON'T WANT any debris?
Or, maybe, if you were considering the Holothurid as a refugium inhabitant....consider it's feed requirement rate VS your "waste" production rate AS IT'S FEED SOURCE.
~ will you have enough DIRT/DEBRIS/DETRITUS for it as an efficient sand-sifter?
~Do you even WANT your substrate to be sifted?
ALL Holothurids produce varying levels of toxins, so should only ever be restricted to maitenance of relatively small species.
Weigh up the benefit-loss scale.
Some are shy and withdraw tentacles, at slight disturbances...missing meals, and starving to death...dangerous!!!
Here's another caution...
"It is vital that sea cucumbers only be associated with peaceful unmolesting fishes and invertebrates."
[Baensch Marine Atlas 3]
by Dr Robert Patzner