What you are seeing is normal in unmature systems and the reason why I paint my backgrounds black rather than blue. I don't clean the back and with black you don't see it, with blue like my sons tanks have it looks nasty.

A reef will continue to evolve and go through algae stages for up to a year or more. I remember when I first set up my present system 7.5 years ago, it went through every stage you can imagine before settling down. I have never used anything but RO/DI from a well maintained Spectrapure MaxCap so I know none of it was water quality related yet I still had it all and in every color of the rainbow. I went through hair algae, diatom blooms, dinos, bubble algae, cotton candy algae, red slime, green slime, black slime, you name it I had it at one time or another and chalk it up to the maturation process.