well the only reason its imbedded is because of how your rock is, but what i have read if your touching it and its not coming off easy it could possible be coralline, im just a little iffy on it because i havnt seen it like that before but it could be the start? but if its not coming off easy and not dusting away and breaking up easy when you touch it, it may very well be coralline. Just keep and eye on it and watch it grow, the more it matures the more you will be able to tell what it really is.