While at times the number of legs on a starfish can determine exact species, IMO, these are just the the common Striped Miniature Brittle Starfish, which belongs to the
Ophiactis spp. family of starfishes.
They sometimes have six arms because one of their methods of reproduction is by asexual fission. Good to have in your tank, although can get to dense numbers if tank is heavily fed and foods uneaten by your fishes.
I have these in an isolated 20G tank with some Bicinctus clownfishes and live rocks. Their population increases as I feed more. I don't have any in my other tanks as I started with different rock from the other tank.
Best,
Ilham
