I agree with Drew.
They are fine in a reef, assuming you have a decent size tank with a good amount of LR (they 'graze' on microbial/bacterial films, and perhaps other encrusty stuff you don't pay much attention to). If not, the star will suffer, but it shouldn't bother anything else. I would not add any other reef safe seastars however (not including brittlestars or serpentstars), unless you have a tank over 75g+ with at least that much LR. There is another thread on these guys where they have 3 in a 75, and they have kept them over a year...which I consider a success. The one year mark is key. However, this applies to this species only. Other reef safe stars belong in tanks over 100g, IMO (for best chance of success).
It seems to be hardier than other species of
Linckia (hence it survived being a hitch hiker on LR and probably not a very long acclimation). Watch for signs of disintegration within the next month, which means it is dying from acclimation stress. This group of stars in not very tolerant of changes in water parameters, so be careful when doing water changes (which you should be doing anyway, so no biggy). Many die at or within the 1 year mark, from starvation.
Generally my suggestion is to let the star add another- by dropping an arm- rather than introducing another on your own. Best to let them decide if there is enough food to keep them happy and healthy enough to regenerate (and sustain another star).
Arm drops are pretty cool. If you see the one with a shorter arm, and a little stub wandering around the tank, that is what happened. Both will regenerate the missing parts. The smaller will become what is called a 'comet' star, meaning it has one really long leg, and 4 tiny nubs for awhile. Very nice way of getting tank raised seastars
Maybe a way to convince my hubby of, hmmm, the need for a 1000g+ tank in the future?