I ran into something along these lines about four years ago
A buddy brought me back some black sand from Costa Rica.
It was about 5lbs so I just added it to my existing bed to provide some cool color patterens. Later that same week I was cleaning the tank when my magnet fell off going around a corner, and that was when I discovered the day from hell. I picked up the magnet to find that it was completely covered by black shavings. Another buddy of mine is a 15+yr machinist so I called him for some advice. My buddy told me that black or even white sand can contain high amounts of Iron if it's collected from a vollcanic region. Since any type of metal/saltwater in an enclosed system is not a good thing, I made up my mind that same day to do a full tear down. I didn't save any of my sugar sand either as I didn't want to take the risk. I just spent the $40 and bought another bag of sugar sand. After I was finished I would say it took me about 14hrs from tear down to setup, not to mention new water and rinsing all the live rock with a turkey baster and a few more buckets of saltwater.
If I were you, I would do the same before you start getting some serious problems.
Best of luck, Rich