It is not aptasia or a feather duster. From what i can see from the pic it is probably either a red macroalgae or a sponge. With a closer shot it could probably be identified. If those three central stalks have tubular openings in them, it is probably a sponge. If it is plantlike (for lack of a better term) it is probably a macroalgae. Whatever it is, leave it alone, as it is not detrimental. Just one of the reasons live rock is so great.