From FrugalReef

Coralline Algae is beautiful algae that is usually welcome in any saltwater aquarium. It comes in many colors and encrusts the surface of the rocks (And often plumbing) in your aquarium with a colorful opaque coating. It comes in many shades of green, purple, white, pink, red and orange.

This algae encrusts live rock and prevents other algaes from growing. In addition, it releases chemicals that suppresses the growth of other algaes aquarium wide.