I have some Garf grunge in my tank, and this stuff is loaded with, Pods, co-pods, and and three legged star fish, perfect for the Harlequin shrimp,It takes a couple of months, but my tank is loaded with this good stuff. I also made only a little section of rubble in my tank as I do like the sand look rather than the rubble look.. Most people on this site will negativly dissmiss the grunge and say it has no pods and ect.. absoulte lie, 5 tanks I've done with the grunge all with great sucsess , but all of them takes a couple of months for you to notice life, as there microscopic life forms.. and great for reef tanks!!
so if your infeasted with 3 legged star fish a couple of Harlequin shrimp will be a good addition..and its pretty cool to watch them eat too. and yes they only feed on live star fish..