More than likely what you are seeing are snail eggs. When you did you water change you stated that you moved the power heads and they streached out from the rocks. The power heads dislogged them and they drifted out wards. If you have ever taken a close look at snail eggs, you will see that they are strung together on a slimy string. Tipically not the case with fish eggs, and coral reproducing, well coral release into the water with out being connected. Much like that of a spore on land releasing into the breeze. Brisstle worms you will find do not lay eggs on the surface of a rock, but rather deep inside a rock or in the substrate.