Snails might help a little, but when they say there one day gone the next they, its true. After i cycled my 120g i got a huge diatom bloom and then one day i noticed it dicipated and by the next day it was gone just like that.
If the white stuff came after you put the lights on it might not be sponge, because sponge likes shade so it might be something else. And the feather dusters stay small just because thats the kind they are. The big one you can buy are a different type. I have tons on my rocks and there is more all the time so they seem to multiply also.
I think if there is feather dusters on the rock that some of the life that was on it might not have died to begin with so it should be fine. I had a few pieces of rock that i left in a 10g tank with no filtration and no water changes for about a month. After evaperation the salitnity of the water was through the roof, but within a week of putting it in my new tank there were featherdusters coming out of existing tubes on the rock so nothing surprises me anymore when it comes to life on lr