Taken from a member off nano-reef
"Coraline algae can also be green! It probably started to die off when you first brought it home and now is coming back! The back tank of my tank has as much green coraline as pink! The key is to feel it if its hard not soft and doesnt come off easily its probably coraline!"
So i guess it was coraline algae
and for search referance for other begginners
http://www.algone.com/coralline_algae.php