i think there called "flat worms", somebody correct me if im wrong. they are harmless however and they just extend to feed normally. i have a few that i find every now and then, moving the sand around a little bit.
now if its a very small baby bristle worm, then its also mostly harmless and will not cause any real threat as of yet. When they get very large and mass produce is when they can become bad news.
either way, id not worry right now. your tank is still rather unaged i believe from other posts, so id just let it be. if i can remember the proper name for them ill chime in, or hopefully someone else can correct me, i have a feeling thats not there common name.