What's in My Reef?

Hello friends,

My reef is 6 years old. I haven't added anything to my tank in almost 2 years. Cleaning my tank today, in my overflow box I see these "octopus" looking things. they are about the size of a quarter/half dollar when fully extended. i was looking for a comparison and seen a bristles star. Is that what they are?

Hope the pictures are clear enough. They just appeared in my overflow. I have about 6 that I can see.

Next, are they harmful?

Thanks friends,

RJ