One of the dreaded algaes we see in our tanks is called "Red Slime Algae" but it is actually Cyano bacteria. Typical solutions: reduce Nitrates & Phosphates; increase flow; siphon out all you can see with airline tubing. Use a turkey baster to blast this off of sensitive corals, because it will smother them to death.
This long spined urchin is a Diadema Urchin. I recently purchased this for my reef because I find them to be beautiful. They are known to help mow down green hair algae, but urchins often consume coralline algae as well. This one is quite small, as you can tell by the neighboring zoanthids on the left. The yellow thing in the center is not an eye, even though it does seem to be looking around.
Sometimes you will find a very dark red growth encrusting on the rock. This would be red Coralline algae, and it is nothing to worry about. Rather than being cyano bacteria, it acts like it is painted on the rock and won't budge when hit with a powerhead or turkey baster. The more common type is pink coralline, but some red mixed in can look quite nice.
This very cool little creature is a Lettuce Nudibranch. They come in specifically to help mow down nuisance algae such as bryopsis (visible in the foreground). I purchased six of these for my reef tank, and each was about 1" long, and about 3/8" wide. Delicate and very light, they were introduced into the tank with the pumps off. Each one was held gently against the rockwork for about 10 seconds until it held on. Every day, I'd find one or two of them in the sump or refugium, and I move them back up to the display tank. Covering intakes to powerheads would be wise.
You've probably seen the bright yellow and black Foxface. This is a more expensive and somewhat more rare Magnificent Foxface (Siganus magnificus). This particular fish is a ravenous herbivore, and will mow down nuisance algae in a reef. When stressed or frightened, their color changes. Special care should be made to avoid the spines on its back, as these carry a serious sting. Reef-safe.
Padina is a neat looking scrolling algae. It doesn't do any harm, and adds some shape to your reef. This particular leaf grew for weeks at the local fish store, getting larger until one day it was gone.
Over the past year, more people are keeping this beautiful macro algae to export nutrients from the water. It's likely Halymenia sp.
Photo by Brian Grippe
In nature, the Mangrove plant grows in brackish water, providing a safe haven for newly hatched fry (baby fish). The plant absorbs salt water, and is able to extract freshwater for growth. A benefit is that it will hold silty soil in place, and act as a filter as water runs through its roots toward the neighboring reefs. Some reef keepers grow these in their systems to help decrease nitrates while providing an attractive plant to their display. The leaves must not touch the water. These two shoots were planted in the refugium of my 55g reef.
This is Pink Cotton Candy algae, which is a major pest in some aquariums. Sandra Shoupe wrote an article about this stuff if you want to read it. The huge Mexican Turbo Snails will devour it, but they can be bulldozers with your corals as well. This algae is definitely a pest in our aquariums. Not Reef-safe.
While visiting a reef keeper in Austin, I saw this interesting macro algae, nicknamed Red Dragon's Breath - Halymenia sp. What makes this a nice choice for some tanks is that it adds a splash of red color, and the plant doesn't take root to the substrate nor the rockwork. Some fish may eat it, so if you are getting it for looks, try a small piece in your tank first to see if any of your livestock devour it.