Aiptasia is a pest anemone that can take over your reef tank if you don't get rid of it. They can be brown, yellow, clear, or a few other colors. If you aren't sure if something is aiptasia, poke it with something. If it sucks into the rock, it is likely aiptasia. If it just folds up, it is probably a harmless polyp. 
BUTTON POLYPS
These polyps are very common hitchhiker corals in a reef aquarium. They come in a variety of colors, but are commonly brown or green.
These little worms are extremely common on live rock in a reef tank. They are beneficial filter feeders. The worm lives in a tube and uses its feathered crown to catch particles from the water. They will retract the crown into the tube when spooked. They can be colonial, as in the photo to the right, or can be individual. They come in a variety of colors. with lots of great pics.
SPIONID WORMS
These worms also live in tubes, but rather than filter feeding like the dusters, they will grab larger food with their palps (the antenna-looking things). They are good scavengers.that includes spionids.
VERMITID SNAILSThe vermitid snail is yet another critter that lives in a tube. These filter feed by throwing out a mucous "net" to catch food. Most of the time, you won't see the worm but will be wondering what the "spiderwebs" in your tank are. Here's your culprit. They are harmless but can irritate corals if their nets are too close.
SPIRORBID WORMS
Little hard white spirals on your rock and aquarium glass causing confusion? When you look real close, do they have little feathers sticking out the end? Congrats, you got yourself some spirorbid worms. These little guys live in their calcareous shells and filter feed. They are harmless additions to your reef.
FORAMINIFERANS
These little filter feeders are very common on live rock in a reef tank.about them.
ZOANTHIDS
While some of us pay the big bucks to buy these pretty soft corals, some of you get lucky enough to get them for free. While most hitchhiker zos are brown, colorful ones have been known to sneak in on live rock.
MAJANO ANEMONES
These pretty little anemones come with a bad attitude. They can quickly reproduce and will sting anything they come into contact with. Therefore, most reefers choose to eradicate them. Too bad... they really are pretty.
COLONIAL HYDROIDS
Hydroids come in many shapes and sizes. If you have something that sorta resembles a feather duster, but not really... look into hydroids. They can pack a mighty sting so it is good not to let them grow next to corals. They can be brown like these, white, pink, or other colors. I have been unable to find a good hydroid article to post, but 
DIGITATE HYDROIDS
Digitate hydroids spread quickly and sting anything they touch, making them nasty to have in your reef tank. Too bad as it is cool to watch them bob up and down as they stretch out from the rock. They are white hairlike critters with a bulbous tip which will mostly come out at night.
SPONGES
Something soft and squishy on your rock? It's likely a sponge. They come in a wide variety of colors, shapes, and sizes.with lots of pics.
TUNICATES
Tunicates (or sea squirts) are another filter-feeding hitchhiker. These vase-shaped critters can be clear, purple, orange, or a variety of other colors. I am looking for a good article on these guys as I don't know much about them.
BARNACLES
If you see a little raised hole on your rock with feathers quickly going in and out the hole, you've got a barnacle. It is actually a little filter feeding shrimp-like creature that lives in the hole.