We have a 29 gallon saltwater tank. recently we saw this white micro little thing. I has a disc center and 5 legs. at first i thought starfish...but it moves its legs like tentacles...what is it??

we have started seeing all kinds of things in our tank since puchasing a big peice of live roch...even a clam! we are new to saltwater so we're learning as we go...but I cant figure out what that little white guy is...