Causes of undesirable Algae:

The main cause of algae in your system is the source of light + excessive nutrients (phosphates, silicates, nitrates). If you have algae beginning to growing this may be a good sign that your tank is biologically stable. However if you observe too much stringy algae cluttering any livestock, there may be something out of tune with your water quality. There are two major types of algae, Micro-algae and Macro-algae. Micro-algae is a thin film that coats the glass of the tank which are microscopic plants. Macro-algae is visible and comes in various types of color usually brown, green, dark brown/green, or red. These types can easily be cleaned off with an algae scrubber. Here are some different types of common aquarium algae:

Coralline Algae (Good Algae)	Coralline Algae
This type of algae grows in locations with an abundance of calcium, iron, iodine, in addition to intense lighting. It is a good attraction to the tank providing a great purple/ maroon color. It is a good sign that your tank will be able to handle corals and other sensitive creatures if you have strong lighting and good water quality. This algae is not bad and contains important bacteria and enzymes which break down waste. This algae competes for space against nuisance algae which can be beneficial. Coralline algae is made out of CaC03 (Calcium Carbonate) which forms a hard calcified structure and can sometime be hard to scrape from glass. One would typically witness this algae in a tank with plenty of circulation, calcium additions, and reef-like lighting.
Brown Algae (nuisance)	Brown Nuisance Algae
This type of algae is quite common among beginner aquarists who have just established their first aquariums. To reduce the amount of algae, make sure phosphates are kept below 0.1 (at 0.03ppm) or you can use Astrea snails. These snails are very good critters to have if you wish to have your algae cleaned off rapidly. Most reef aquarists generally keep around 2-3 per gallon of water in order to keep the tank clean. You could also just use an algae pad to rub off any unsightly algae. It deteriorates water quality so make sure you are doing your periodical water changes with phosphate, nitrate, and silicate free water.
Green Algae (nuisance/stringy)	Green Nuisance Algae
This algae is closer to bacteria than other algae would be considered. This type of algae is very common to the saltwater aquarist and is not harmful unless out of control. This algae may be a good source of food for grazing fish such as the tang/surgeon species. This algae is a sign that your tank has matured for a good amount of time or your lighting has been just upgraded. You want to clean as much of this off as possible if you don't have any algae grazers. The green algae in this picture is really hard to get off glass especially off of liverock or tank decorations. I would suggest using a plastic razor of some sort to get it off.
Controlling Unwanted Algae Growth	Reverse Osmosis Unit
Algae does best in water supplied with a good source of or accumulation of nutrients. To try and control the growth of algae make sure you add as little nutrients as possible which are useful to the algae. Algae also does better where there is no flow or current of water. To keep the algae away you could increase current in places of your tank or increase the total overturn rate of your water volume. Many marine aquarists face the problem of having a sudden bloom in algae caused by lack of periodic water changes or a sudden addition of nutrients. Doing water changes and siphoning detritus from rocks and gravel in the tank may help reduce algae growth. One method commonly used by aquarists is the use of a reverse osmosis unit in order to replace evaporated water with nutrient free water. In most cases, detritus buildup is usually the cause for uncontrollable and overwhelming algae blooms which can easily be solved by: 1) Periodic water changes 2) Use of Natural Herbivores (snails, tangs, gobies, starfish...) and 3) Detritus buildup removal.