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Then you have asternia stars. Congrats. They multiply like crazy at times. The numbers seem to ebb and flow. I have some with just 1 stub of an arm and 1 or 2 with 6 arms.
They reproduce by dropping off arms. Melosu58's picture shows them full size.
Mine came to me on a piece coral. At first I tried to removing them. Now when they get too overcrowded I move as many as I can to my predator reef tank.
I dont know anywhere you can buy them. They usually hitch hike on LR. If you have a LFS nearby go to his LR bin and you`ll probably see some in the tank. He might even give you some.