I believe it to be a blasto as well. I have one that came in on a rock of zoas like yours,it has become one of my favorite corals. I was moving the rock one time and it closed on my thumb and it was extremely difficult to get my thumb away from it. Mine was a bit larger than yours,about the size of a quarter with really nice pink highlites. It has since split into two different mouths and it looked like it was trying to split again earlier tonight but now its back to just two. A very peculiar creature to say the least.