Harlequin Shrimp - Hymenocera elegans - are gorgeous creatures. I've never owned one myself, but was able to get a good picture of one while visiting a LFS. These shrimp only eat starfish, and will otherwise perish in our tanks. If you choose to own this shrimp, you'll have to buy a starfish or two each month just to keep it well-nourished. There is a chance it will eat the tiny white starfish (Asterina), but since I've got several Serpent starfish already in reef, I'm not willing to buy this particular shrimp to see if that is true.
Check out this really cool starfish - a Pillow Starfish! I'd never seen one before, nor heard of it being referenced. The local fish store employee smiled and said "You don't want that in a reef tank." 'nuff said. Not Reef-safe.
WOW! Beautiful! However, this is not a good addition to your tank. The Sea Apple - Pseudocolochirus violaceus - is a very interesting creature to observe as it filter feeds from the particulates flowing through the tank, doing so methodically and beautifully. However when (not if) this creature dies, it may very well take out the rest of your livestock at the same time (depending on tank size). NOT Reef-safe
Most hobbyists encounter a strange calcified tube with a web coming out of it, which is usually noticed during tank feedings. The web is cast by a Vermetid, a snail that uses the web to capture particulate-sized foods in the water column. Using its jaws, it consumes the web with the food attached. While it is common to have a few, Hop discovered that his tank was the perfect breeding ground for thousands of these creatures, which led to the demise of his reef. The webs irritated nearby corals, often shutting them down entirely.
The orange worm is a predatory worm that needs to be extracted once discovered in your reef. It exudes a toxin to stun its prey, and devours it. If spooked, it retracts back into the safety of the rockwork via that same toxin-slime that it builds in its wake. It will appear to be bright orange when spotted in the tank during flashlight duty. Avoid shining the light directly on this worm if you hope to catch it, and have forceps handy.
The Red Serpent Starfish is a good choice as a clean up crew member. It works its way around the tank, disturbing the sand and looking for wasted food and film algae on the rock and glass. Starfish typically should be acclimated slowly over a two-three hour period, using drip acclimation, an airstone and a small heater to maintain oxygen & temperature during that session.
If you happen to spot this guy on the glass, do not fear. It's a Spaghetti worm out for a hike, trying to find a new spot to camp. Normally you won't see anything but a bunch of tentacles spreading out from a central point in the sandbed, but in this image you can see the entire worm with its tentacles all bunched up against its body. I have another image where I was feeding one, which I will try to find and edit this entry.
Early on in the saltwater hobby, I found tons of bristleworms in my tank. I really didn't like them, as they came squirming out of the rockwork and substrate at every feeding. So I bought a Longnose Hawkfish - Oxycirrhites typus - to take care of the problem. And this fish did just that, he got every one in sight. However, it didn't decimate the population, because there are always more deep in the crevices of the rockwork. I just don't have to see them, unless I look late at night with a flash light. This fish likes to park in odd spots, like it is On Duty.
Although they come in many sizes, featherdusters are a popular addition. Larger ones called Hawaiian Featherdusters are very common. A worm resides within a tube, and extends a crown of "feathers." These are used for filter feeding. Although they don't need light to live, feather dusters quickly retract within the tube, pulling the crown in with them when they sense danger due to a sudden change in lighting or current. Feeding phytoplankton every other day suffices.
This tiny creature is a Flatworm - Amphiscolops sp. This type is virtually clear, and quite small. The rounded end is inflated like a sail, and stretches in the direction it wants to travel, then like a suction cup it grabs the glass and pulls the rest of its body that direction. This is a benign type and you shouldn't worry if you see any in your tank. I've seen them on and off in reef tanks as well as refugiums, usually short-lived and forgotten later. Reef-safe.