Most algae is not that attractive, and is not very useful (unless you are a hermit crab).  Green algae makes your rocks and aquarium walls look like a Chia Pet.  Red slime algae looks nice, but it gets everywhere.  There is, however, one type of algae that aquarinists love to see: coralline algae.

Pink Coralline Algae on live rocks, pebbles, and shells

The coralline algae in my aquarium is pink.  I understand it can also be purple, yellow, and a few other colors.  But the main difference between coralline and the more pesky algae is that coralline algae is hard.  Apparently coralline algae has a lot of calcium salts.  I had a couple of small spots appear on the front and side walls of the aquarium the other day.  It took some effort with my thumbnail to get it dislodged.

Pebble with coralline algae spots

The photo above is one of the small pebbles, about 1 inch across.  You can see a number of pink spots on it, contrasted by a slight haze of green algae.

Why would coralline algae be welcomed in a marine aquarium?  Simply put, they consume nitrate.  Since nitrate is the third and last stage of the process of breaking down ammonium, this is a very welcome trait.  I check my chemical levels once every week.  For the last 4 weeks my levels of ammonium, nitrite, and nitrate, have all been zero.  I credit this to the increase in coralline algae in the aquarium.