What is this?

[diy]

I always see this stuff while browsing pics of reef tanks. What is it and why don't people clean it off?

 

[9supratt4]

It's coraline algae. It is signs of a healthy tank. Some people like the look of it and say its natural....others scrape it off the glass. It's very difficult to remove from plastic.

 

[diy]

Thanks man, I have always wondered what is was.

 

[tiff*]

I'm in the process of cleaning it off my glass what a pain in the %$^ it is to scrape off