What is the green stuff on the back of the tank?

Looks like a different species of coralline algae. It comes in pink, green, red, maroon, purple, neon orange (and I'm sure other colors as well).

 

+1

It also comes in baby poop brown as is evident by my tank unfortunately...

 

What is the purpose of coralline ?

 

A lot of people like the way it looks. It can make it more difficult for nuisance algaes to take hold.

 

It's also a sign of a healthy reef (assuming you can maintain it too).

And you can also find it in shades of teal/baby blue, in addition to the colors Curt listed. Pink/purple are the most common once your tank is established.

 

Thank you, I was actually wondering what this bright teal stuff was that was growing on my rock.

 

Kristiavi, is it hard or soft? Light or dark teal? Dark teal could also be cyano.

 

Its bright teal and I havent tocuhed it so i couldnt tell you, but the rock its growing on is covered in coraline(dark purple, bought it that way) I have also noticed the neon teal color on the shells of a few snails.