hmmm...well...
I think the only Asteroidea that can go in a tank that size, is either a more ferocious star(CC star, Red general star, or something along those lines), or a tiny pale asterina (well, i am sure that somewhere on earth there is an easy safe colorful star, but this "dream star" isn't available to us meer mortals).
Now when you take into account that it is a seahorse tank, then you have now limited yourself down to just the asterinas.
Really the only group of Echinodermata that I could think of would an urchin [tuxedo urchin, or something similair, but stay away from flowerpot urchins, fire urchins, diadema (I think all the hype about the danger these pose to seahorses is undeserved, but in a tank that size, the urchin will probably starve, and they also get so bug that it probably would be a very large concern that they would impale your seahorses), or pencil urchins (they are predators, on sessile vertebrates, and invertebrates, and are considered a threat to a sleeping seahorse)