fern looking things that I have had are usually caulerpa algae but this has been contained to the LR and sand near LR. If this is caulerpa do not worry too much at the moment as your tank is young, however, keep watch and do not let it take hold as this will spread fast, it does need food and starves quite easily. If you do have caulerpa and it is only on a few rocks I would remove them and scrub them clean it looks great but is a real pain to remove once it has taken hold. a cycling tank is also a great catalyst for nuisance algae's but also a great killer of them too, this is where time plays its part