Sounds like a little brittle starfish and regardless of the ID
, it's tiny...it can be anywhere...maybe it was eaten...maybe not. If not...no guarantees you'll ever see it again.
A photograph of those tentacle like things sticking out of the rocks would be helpful. Most likely another starfish...could maybe even be a fireworm or bristleworm.
Watch the tank at night. You can use a red light to observe the tank without disturbing the night time cycle. There's a whole other set of life taking shape. You may get to see these mysterious critters at night come crawling out of their daytime hide outs.
Have fun discovering your tank
...It's always good to make a new topic and keep the 'thread' to it's subject. A thread is the line of replies that follow within the topic post. When you have a different topic to bring up, create a new topic. You're doing good LOL
We, as a people, know so much more about outer space than we do about our own oceans. This lack of knowledge can very well spell the dangers that lay in wait for us.
The oceans surely would swallow us before a rock comes down to smite the planet of it's life.