I didn't beleive they were autonomous. But it lost a hunk of a limb that has been moving around for close to a week now. I'm hoping it grows limbs.
For clarification, there are two different concepts you're thinking of here:
1 - Autotomy (not to be confused with autonomy) - this is where an animal drops a limb (like an arm or a tail), typically to distract a predator.
2 - Fissiparity/Fissiparous Reproduction - this is where an animal splits into parts (typically, but not always, in half), and those parts both regrow into whole, separate animals.
(Aquilonastra starfish - known in the hobby as Asterina starfish - are great examples of fissiparity, sometimes splitting in half, and other times just dropping a single leg; in either case, the dropped parts grow into new starfish).
Brittle/Serpent Stars (Ophiuroids) of various species may exhibit one or both of these behaviors.
If just the arm is detached and there's no part of the central disc attached to that arm as well, then I expect the arm will eventually stop moving and decay, but I don't know for sure. Personally, I'd keep it to see for sure what happens to it over time.
Either way, cool to see, but possibly cause for concern if you don't know why the arm autotomized:
"experimental studies revealed that autotomized arms continue to function several days after they have been dropped from the organism. Not only do they continue to move, they appear to have the capability to find their way under habitat and shade just like the whole organism would do in the wild if it was fully intact. Although not a lot of research has been done on the fate of the arms after they have been dropped, one potential reason they can still move days after being released from the organism may be to confuse predators into thinking that it is the actual organism itself giving the brittle star time to successfully escape."*
*Source:
