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I suspect they were hitchhikers from a piece of live rock, and you just happened to catch them. Also, if you happened to seed your sand (if you have substrate) with a scoop from an established tank, then they can arrive this way as well.
Starfish may reproduce either sexually or asexually. When asexual reproduction takes place, the animal breaks itself into two pieces. From these pieces, the animal is able to regenerate new body parts, eventually becoming whole again. Regeneration is also used when a starfish loses an arm, which is a fairly common occurrence.
You might have a small micro-brittlestar or you might have an asterina starfish. The asterina's commonly reproduce asexually in reef aquaria.