jeremy,
people always think that algae = bad.
this is not always the case. yes some are bad such as hair algae and bryopsis. they can get very prevalent and usually an eyesore.
but some growth of algae is always reccomended around the tank and on rocks. green film, small turfs of algae are all ok so long as they don't grow out of control and smother other things.
with correct nutrient levels, these algae can grow hand in hand and not overwhelm everything.
algae is good. it's food for the critters in the rocks, pods, fish, and also create a very natural look to the tank. it improves O2 saturation and remove CO2 build up, it provide homes for pods as well. tangs and dwarf angels rely heavily on these algae in the wild and having such algae growth can even do away with the need of feeding them prepared foods in our tanks.
all purple and brown and no green will look far too weird and very funny. in fact, solid purple coraline encrusted rocks are unatural. it's because of the high and consistant levels of calcium and other elements in our water for our SPS, that's why the coraline over grow.
in the sea, coraline don't cover everything. the rocks are laden with green algae, corals, sponges, foraminiferans, and a whole bunch of other things. it's a living breathing structure with a million things on it. in our tanks, only coraline.