Could you help me identify this star shaped glass attaching guest in my tank?

[Gore15]

I was inspecting the tank and saw this star thing in the tank. I wonder if this is a mean pest or it can be a good addition to the tank. For now I am keeping it in a separate glass container in the  filter area of the tank to keep it in good temperature and safe until a I know more.

 

Edit: The size may look big here but it is less than 1cm in length and almost flat in the back.

[Ratvan]

Asterina star fish, some people say the eat Zoas. I've not had an issue with them in my tanks

[mcarroll]

Free cleanup crew.  They are all over my tank.  😊

[king aiptasia]

Asterina, they sometimes rip themselves in pieces to make more friends for themselves

[applejk]

if you want something to eat it you could always get a harlequin shrimp but they only eat starfish so you'd have to sell it once it gets rid of all those Asterinas.

[thecoralbeauty]

most likely they'll be helpful- they do clean. I had some that multiplied to massive proportions over the course of 5 years and i had to get a harlequin shrimp to mow some down. I did catch them eating zoas once my populations got large enough. 

[Gore15]

Thank you all. I let it back into the tank. My tank is pretty small so if I see more of them I will try and do population control 😛

[rough eye]

one day i was like "aw look a pineapple sponge."  and the next day "aw look an asterina star where the sponge used to be - what? GOSHDARNIT!"