I just recently noticed the same thing growing on some of my live rock. I thought it was justed coraline algae.
My bulbs were right at a year old so I thought maybe that was the cause. I've since replaced the bulbs and haven't noticed any new growth (white spots that is).
if you look very close, do they look like little curly Q's? if so, they are little fan worms. they won't hurt anything. like little baby featherdusters.