Those are brittle star arms, I usually see them all over my tank when I feed, at night in the moonlight they come out one once I came home they were all out and crawling all over. I grabbed the test kits and checked everything was fine, I didn't know I had was going on so I started googling. There they were having a brittle star orgy right in front of my eyes. I don't know if it was the temperature change or just what that could have brought it on. I know when I moved my rocks to my 24 gallon one crawled over my hand that was probably 6 inches tip to tip. There are several different body types and colors I have even seen them nearly blood red.