base rock

I have 4 pieces of Real Reef dry rock left over from a recent tank build. None of the rocks have been submerged in water so you will need to rinse them thoroughly before using. Total weight is 5.4 lbs. and the photos show the actual rocks available for sale. The biggest piece is a little over...

I found this black stuff on a new piece of CaribSea South Sea Base Rock (it came out of the box a few days ago and has never entered my aquarium). It does not scrub off and appears to be encrusted on the rock. Should I worry about it?