a surrogate host is just another coral that the clown chooses to host. what would be a good choice? well according to GARF, clowns are not picky and will pick a variety of stuff to host like shrooms, Brain Corals, Elegance Corals, Xenia, etc... but the best surrogate host by far is sacophytons (aka: Toadstool or Gold Crown or leather coral)
http://www.garf.org/39/fish/pict.html