maybe it was bad quality sand that had some substances is like small particles of wood that look like sand mixed in with the rest.
but if it makes you feel any better iv got some of my own but im pretty sure its excrememtn from the snails. dont worry too much about it unless its causeing serious problems for the entire system.
iv learned that every slight problem will either go away in time and solve itself, or some slight adjustment in your system fixxes it and as long as your corals are alive keep on top of your water chemistry not the mehanical mixxtures within, and as long as your corals are alive chilax. (this doesnt work for fresh)
hope this helps