Yes, it does look like an asternia.
Advice - take them, I say them because if you have one, you have many. Look at your tank in the muddle of the night, they come out in numbers then. I have made a game out of killing these buggers, I call it my star hunting hour. In the beigining on a good night I would lance (With a baboo skewer for shishcabobs) and place them upsidedown at the top of my tank to watch them die!
I tell you this because the first time I saw one it was munching on some of my corals. See if you can see some with different size legs growing.