Things they could be: feather dusters, digitate hydroids (not nice, but something you can deal with) and ---because they have a full disc, not a barnacle type---too feathery for a baby anemone (majanos and aiptasia, neither good, but nothing catastrophic), and other things mentioned. They have to grow a bit before they'll be easy to identify. Personally I let live rocks be live rocks with most anything they bring in, and rely generally on good water balance and proper lighting to keep bad things in check. Most of the really obnoxious and common plagues are green (algaes and plants). There aer a few animals not so good. But not too common, either. Observe your tank after dark with a flashlight, and you may see more life.