while it's not unusual for some small amount of "black dust" to rinse out from Activated Carbon, it should quickly clear ... AC is meant to be particulate not dust
If you can't return it, then you could use a micron bag (various sizes sold in the marine section) that should keep the "dust" inside the bag.
Though this is likely very low grade AC so I'd not bother with it, again most AC sold for Marine seems to have better quality control
