Macroalgal beds are composed of 3 different catagories of algae. They are green (Chlorophyta), brown (Phaeophyta) and red (Phodophyta) algae. What makes a it a macroalgal bed is that it is visible to the human eye. Macroalgae are types of plants and therefore convert sunlight into chemical energy via photosynthesis. These the different colors of these plants are based on the pigments that they contain. As you likely already know chlorophyll pigments give the plant a green tint. Chlorophyll is found in all types of the macroalgae. Red algae also contain red and blue pigments called phycobillins. While brown macroalgae contain orange pigments called carotenoids.
There are many species of macroalgae but only 4 are classified by the National Oceanic and Atmospheric Administration (NOAA) as being capable of growing dense enough to form a macroalgal bed. These are Ulva spp., Gracilaria pacifica, Fucus gardneri and Sargassum muticum. 
Macroalgal beds can play a beneficial role in aquatic habitats. They can provide food for anthropods, other invertebrates and herbivorous fish, a substrate for gulls to make nests out of and a habitat for small/ developing fish.  Macroalgal beds also play many beneficial roles in shallow estuaries. One such role is the habitat for juvenile Atlantic blue crab. Atlantic blue crab among many other species use the complex maze of algal to hind lay eggs and provide an introductory habitat for their offspring.
Macroalgal beds and macroalgae in general are the known to utilize excess nitrogen commonly associated with agricultural runoff. In this way macroalgae beds are the mechanism that allows the the oceans to handle the anthropogenic effects associated with our current agricultural practice. Macroalgae stores excess nitrogen for periods of low nitrogen availability. The nitrogen storage capability of macroalgae has potential to be a source as an agricultural fertilizer.
Impacts on Coral Reefs
The ecology of coral reefs is impacted by the roles played by macroalgal beds. An abundance of macroalgal beds are typically detrimental to coral reefs but a lack of macroalgae can also be detrimental to coral reefs. Therefore finding a balance of macroalgae is important to the overall ecology of a coral reef ecosystem. The key beneficial impacts that Macroalgal beds provide for coral reef ecosystems include primary production, organic carbon leaking in water, nitrogen fixation and structure/ habitat. 
Primary production is the overall conversion of sunlight, water and carbon dioxide into organic matter. Therefore primary production can remove carbon dioxide from the oceans helping to reduce the ocean acidification associated with increasing carbon dioxide in the ocean. Organic matter (biomass) provides the food for herbivores and shelter for juvenile and smaller aquatic species.
- "Mapping Coral Reefs and Macroalgae." Mapping Coral Reefs and Macroalgae. United Nations Educational, Scientific and Cultural Organization, n.d. Web. 15 Apr. 2015. <http://www.unesco.org/csi/pub/source/rs12.htm>.
- "Macroalgal Beds." San Francisco Bay Subtidal Habitat Goals Project (n.d.): n. pag. Chapter 9: Macroalgal Beds. San Francisco Bay Subtidal Habitat Goals Project, 2010. Web. 15 Apr. 2015. <http://www.sfbaysubtidal.org/PDFS/09-Macroalgal.pdf>.
- By, Sponsored. "Understanding the Role of Macroalgal in Shallow Estuaries." (2002): n. pag. The Maryland Department of Natural Resources. The Maryland Department of Natural Resources: Resource Assessment Service, 11 Jan. 2002. Web. 15 Apr. 2015. <http://www.dnr.state.md.us/irc/docs/00015761.pdf>.
- State of the Great Barrier Reef Report. Townsville, Qld.: Great Barrier Reef Marine Park Authority, 2003. Great Barrier Reef Marine Park Authority. Australian Government, July 2008. Web. 15 Apr. 2015. <http://www.gbrmpa.gov.au/__data/assets/pdf_file/0019/3970/SORR_Macroalgae.pdf>.