The Camelback Shrimp is recognizably different from the Peppermint Shrimp, in that it has a hump on its back. This shrimp isn't a good choice for reef tanks, but can be used to devour AEFW off infected corals, according to one site. Set up a hospital tank, place six of these shrimp in the tank, and place an infected coral in their midst for exactly 30 minutes, not a minute longer.
At some point, I received this picture but I don't recall the source. These are pest Flatworms - aeolids - that tend to gather upon the surface of mushrooms, for example. While they may look like guests, they are definitely not helping the mushroom live. it is possible these feed upon the mucous of the coral, and are a general eye sore. They can be carefully siphoned off the object with airline tubing, or you could treat the tank with Salifert's Flatworm eXit. These are definitely unwanted.
Photo by unknown
If you see this enormous flatworm in your tank, siphon it out. It is a predatory flatworm, and its food is often clam flesh. Their pattern is designed to act as a camoflage to match the mantle of a clam, so it can park on the animal as it consumes it. This flatworm can devour snails quite easily. Blanketing its prey as it pushes its stomach into the aperture of the snail, to feed upon the meaty snack. Not Reef-safe.
For those that keep Acropora sp, they've heard of Acropora Eating Flatworms (AEFW) - Amakusaplana acroporae. The best way to identify if your coral has these it to place the coral in a white container with tank water and a few drops of Lugol's Solution. Wait a minute, then use a turkey baster to blast the coral. If you see small oval flat disks flying off the coral, it is infested with AEFW. These are a real threat, and will consume all Acropora sp. in the tank. The above picture points out just how small they are, and that doesn't even include the eggs!
When I bought my 55g tank, it was infested with Flatworms - Red Planaria - which propagate via fission. They are known to increase rapidly, smothering the sandbed and corals. If they all die at once, that crash will wipe out a tank due to the toxicity of the flatworm juice. I vacuumed out 1000's over 10 months, and then dosed my tank with Salifert's Flatworm eXit which removed them entirely. For a time being, some of the local fish stores I visited had these infesting their coral tanks and that left me little reason to buy anything new.
Sundial snails prey on zoanthids, and usually come in on zoa rocks. These snails are easy to spot, and can be removed with forceps.
These nudibranchs eat montipora species. In this picture, that is a big population of nudibranchs, and in the white area (lower right) you can see all the egg sacks. (Photo by Adam DeAnda)
Here are three more pictures from Sanjay Joshi, who recently discovered them infesting a prized montipora colony. Look closely. These are being identified as Embletonia sp.