Regular IO salt. This rock has been in water for 3 months now curing. I have 2 containers right next to each other, one has it the other doesn't . I think I will just drain and refill. I guess it could be something out of rock, but it has been through acid bath, vinegar, bleach, and been curing for months..
When curing rock will start releasing old calcium that is on and in the rock.
That might be it.
When I cure I use a old skimmer to help me remove as much as possible the stuff that comes out or off the rock as the Po4 that will be relaesed from out the rock.