Why were you adding highly concentrated SW though? You shouldn't be adding water that is a highly different salinity directly to the main tank, at least not all at once. This will stress out animals, and if the salt is undissolved, could injure and "burn" them. If you need to raise the salinity for some reason, do so slowly (over hours or days), make sure everything is predissolved. Add to the sump instead. And keep in mind its not absolute salinity that is most harmful, it is sudden changes in salinity.
The duster is just reacting to this sudden change. When they get startled, they quickly shrink into the tube, but if conditions are stable, it'll come out again.
