Can't really make out anything from the pic you've posted but a couple of possibilities are foramaniferans, encrusting sponges or possibley new coralline growth itself.
If adding calcium please make sure that you are properly testing for the Ca
levels as well as the alk
. If unbalanced and additions are not made properly, you can end up with some serious precip. You can look up the balanced amounts >>here
Same goes for the iodine. It must be tested as well to ensure not dosing beyond the 0.06ppm necessary for NSW
. In most all cases iodine is not a required dosing element if doing regular water changes. If you have a refugium with macro algeas or Xenia sp.
then you may need it, but please be careful with it's use.